Patent Publication Number: US-2006003382-A1

Title: Compositions and methods for analyte detection

Description:
The present invention claims priority to U.S. Provisional Patent Application Ser. No. 60/569,716, filed May 10, 2004, the disclosure of which is herein incorporated by reference in its entirety. 
    
    
      This invention was funded, in part, under National Science Foundation Award Number EEC0118025. The Government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to novel methods and compositions for the detection of analytes using the nuclear reorganization energy, λ, of an electron transfer process. In particular, the present invention provides probes of the outer-sphere environment of a redox center and methods of correlating changes in electrochemical properties to characterize the same.  
     BACKGROUND OF THE INVENTION  
      The rate of electron transfer is dependent on a number of factors including the strength of the electronic coupling (e.g., between A and B-H AB ) and the reorganization energy (λ) (See, e.g., Marcus and Sutin, Biochim. Biophys. Acta 1985, 811, 265-322). Numerous electron transfer studies have focused on probing electronic coupling by varying the length and the nature of the bridge (covalent, conjugated, hydrogen bonds, “through space”) between the donor and the acceptor (See, e.g., Gray et al., J. Biol. Inorg. Chem. 2000, 5, 551-559; Bjerrum et al., J. Bioener. Biomembr. 1995, 27, 295-302). Electron transfer reactions are crucial steps in a variety of biological transformations ranging from photosynthesis to aerobic respiration. Studies of electron transfer reactions in both chemical and biological systems have led to the development of a large body of knowledge and a strong theoretical base, which describes the rate of electron transfer in terms of a definable set of parameters.  
      The use of reorganization energy as the basis of detection for biological molecules has been described in U.S. Pat. Nos. 6,248,229, 6,013,170 and 6,013,459. Reorganization energy is the energy required to activate all atoms of the reactant, including solvent atoms in the solvation sphere, from their equilibrium state to the product state, and consists of two parts: λ=λ i +λ o . The “inner” contribution λ i  relates the energy needed to change bond distances and, in some cases, spin state. The “outer” contribution λ o  relates the energy needed to reorient the solvent and is given by eq 2, for the simple geometric assumption of spherical bodies (See, e.g., Marcus and Sutin, Biochim. Biophys. Acta 1985, 811, 265-322). The variables a 1 , a 2  are the radii of the donor and acceptor, r is the distance between them, and ε op  and ε s  are the static and optical dielectric constants respectively.  
               λ   o     =         ⅇ   2     ⁡     (       1     2   ⁢     a   1         +     1     2   ⁢     a   2         -     1   r       )       ⁢     (       1     ɛ   op       -     1     ɛ   s         )               (   2   )             
 
      Detailed studies of electron transfer proteins in which the reorganization energies of the active sites have been determined and compared to those of solvated analogues have shown that proteins minimize outer-sphere reorganization energy (λ o ) of the active site, thus facilitating electron transfer. However, studies of λ o  have not yet been undertaken in which various aspects of the environment of the redox center are systematically varied.  
      What is required are compositions and methods for analyzing the outer-sphere environment of redox center changes in the electrochemical properties that may occur therein in the presence and absence of an analyte.  
     SUMMARY OF THE INVENTION  
      The invention relates to novel methods and compositions for the detection of analytes using the nuclear reorganization energy, λ, of an electron transfer process. In particular, the present invention provides probes of the outer-sphere environment of a redox center and methods of correlating changes in electrochemical properties to characterize the same.  
      Accordingly, in some embodiments, the present invention provides a method of detecting a target analyte in a test sample comprising adding the sample to a solution comprising a compound comprising i) a solvent accessible transition metal complex with a first redox potential; ii) a linker comprising an alkyl chain terminating in a pyridine; and iii) a binding ligand that binds the target analyte; wherein the solvent accessible transition metal complex is covalently attached to the binding ligand by the linker; such that upon binding of the analyte to the binding ligand, a solvent inhibited transition metal complex is formed with a second redox potential; and detecting the second redox potential as an indication of the presence of the target analyte. In some embodiments, the transition metal is ruthenium. In some embodiments, the transition metal is iron. In some embodiments, the binding ligand is a protein. In some embodiments, the protein is a peptide. In some embodiments, the target analyte is a protein. In some embodiments, the alkyl chain is C4 to C10. In some embodiments, the alkyl chain is heteroalkyl. In some embodiments, the linker has the formula (CR2)n-, wherein n is an integer from 4 to 10. In some embodiments, the linker is a heteroalkyl chain from 4 to 10 atoms. In some embodiments, the linker is saturated alkyl. In some embodiments, the linker is saturated heteroalkyl.  
      In some embodiments, the present invention provides a composition comprising a redox active complex comprising a ligand and a transition metal complex. In some embodiments, the transition metal complex comprises a platinum metal. In some embodiments, the transition metal complex comprises ruthenium. The transition metal complex is not limited by the nature of the metal used. Indeed, a variety of metals are contemplated to be useful in the complex including, but not limited to, cadmium (Cd), copper (Cu), cobalt (Co), palladium (Pd), zinc (Zn), iron (Fe), ruthenium (Ru), rhodium (Rh), osmium (Os), rhenium (Re), platinium (Pt), scandium (Sc), titanium (Ti), Vanadium (V), chromium (Cr), manganese (Mn), nickel (Ni), Molybdenum (Mo), technetium (Tc), tungsten (W), and iridium (Ir). In some embodiments, the ligand of the redox active complex comprises biotin. In some embodiments, the ligand is a hormone receptor.  
      The present invention also provides a method of detecting a target analyte in a sample comprising: providing a redox active complex, the complex comprising a ligand and a transition metal complex; exposing the redox active complex to the sample under conditions such that the analyte, if present in the sample, binds to the ligand; and detecting binding of the analyte to the ligand. In some embodiments, the detecting comprises detecting electron transfer between the transition metal complex and an electrode.  
      The present invention also provides a kit comprising a redox active complex, the complex comprising a ligand and a transition metal complex, wherein the transition metal complex comprises ruthenium.  
      Certain preferred embodiments of the present invention use compounds as shown in  FIG. 1 . 
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  depicts the ligands 4-BMP, Bbpy, 4-DMP, and 4-DPEP.  
       FIG. 2  shows the reaction of 4-BMP (L) with [(H 2 O)Ru(NH 3 ) 5 ](PF 6 ) 2 .  
       FIG. 3  shows the target molecules [(4-BMP)Ru(NH 3 ) 5 ] 2+  and [(B-bpy)Fe(CN) 4 ] 2− .  
       FIG. 4  depicts a synthetic scheme to [(B-bpy)Fe(CN) 4 ] 2− .  
       FIG. 5  shows the CV of [(4-BMP) N/S Ru(NH 3 ) 5 ] 2+  and avidin-bound [(4-BMP) N/S Ru(NH 3 ) 5 ] 2+ .  
       FIG. 6  shows the CV of [(4-DMP)Ru(NH 3 ) 5 ] 2+  Ru and avidin bound Ru.  
       FIG. 7  shows the CV of [(4-DPEP)Ru(NH 3 ) 5 ] 2+  and avidin bound Ru.  
       FIG. 8  shows the CV of [(B-bpy)Fe(CN) 4 ] 2− ; avidin bound Fe; and added mediator (4,4′-bipyridine).  
       FIG. 9  shows the square wave voltammograms of (a) [(4-DMP)Ru(NH 3 ) 5 ] 2+ , (b) avidin-bound [(4-DMP)Ru(NH 3 ) 5 ] 2+ , and (c) the result of addition of biotin to the mixture. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The invention relates to novel methods and compositions for the detection of analytes using the nuclear reorganization energy, λ, of an electron transfer process. In particular, the present invention provides probes of the outer-sphere environment of a redox center and methods of correlating changes in electrochemical properties to characterize the same.  
      The use of reorganization energy as the basis of detecting target analyte has been described in U.S. Pat. Nos. 6,248,229, 6,013,170 and 6,013,459, all of which are incorporated by reference in their entirety.  
      The present invention provides certain improvements in the use of reorganization energy techniques, including, for example, particular ligands and linkers, particularly in solution phase assays.  
      In some embodiments, upon binding of an analyte, a change in reorganization energy can be measured electrochemically. Particular embodiments include both the use of biosensors (e.g. the binding ligands are attached to an electrode) or solution phase assays. In some embodiments, the present invention provides methods of detecting the presence of an analyte in crude mixtures. Furthermore, the compositions and methods of the present invention find use in such areas as drug discovery and molecular recognition, as well as other applications. For example, in some embodiments, the present invention provides proteomic biosensors (e.g., an array of redox centers linked to an electrode and to a binding ligand specific for an analyte).  
      The present invention provides methods and compositions for the detection of target analytes using changes in the solvent reorganization energy of transition metal complexes upon binding of the analytes, to facilatate electron transfer between the transition metal complex and an electrode. This invention is based on the fact that a change in the oxidation state of a redox active molecule such as a transition metal ion (i.e. upon the acceptance or donation of an electron) results in a change in the charge and size of the metal ion. This change in the charge and size requires that the surrounding solvent reorganize, to varying degrees, upon this change in the oxidation state.  
      For the purposes of this invention, the solvent reorganization energy will be treated as the dominating component of λ. Thus, if the solvent reorganization energy is high, a change in the oxidation state will be impeded, even under otherwise favorable conditions.  
      In conventional methodologies using electron transfer, this solvent effect is minimized by using transition metal complexes that minimize solvent reorganization at the redox center, generally by using several large hydrophobic ligands which serve to exclude water. Thus, the ligand for the transition metal ions traditionally used are non-polar and are generally hydrophobic, frequently containing organic rings.  
      However, in contrast to conventional methodologies, the present invention relies on the novel idea of exploiting this solvent reorganization energy to serve as the basis of an assay for target analytes. In some embodiments of the present invention, transition metal complexes that are solvent accessible (i.e. have at least one, and preferably more) small, polar ligands, and thus high solvent reorganization energies, are used. Thus, at initiation energies less than the solvent reorganization energy, no significant electron transfer occurs. However, upon binding of a generally large target analyte, the transition metal complexes becomes solvent inhibited, inaccessible to polar solvents generally through steric effects, which allows electron transfer at previously inoperative initiation energies.  
      Thus, the change in a transition metal complex from solvent accessible to solvent inhibited serves as a switch or trigger for electron transfer. Thus, in preferred embodiments, this becomes the basis of an assay for an analyte. Closs and Miller have shown that there is a decrease in lambda in nonpolar solvents in their work on Donor(bridge)Acceptor electron transfer reactions in solution. (Closs and Miller, Science, 240, 440-447, (1988)). This idea also finds conceptual basis in work done with metmyoglobin, which contains a coordinated water molecule in the hexacoordinate heme iron site and does not undergo self-exchange very rapidly (rate constant k 22  1M −1 s −1 ). Upon chemical modification, the heme becomes pentacoordinate, removing the water, and the self-exchange rate constant increases significantly (rate constant k 22  1×10 4  M −1 s −1 ); see Tsukahara, J. Am. Chem. Soc. 111:2040 (1989)).  
      The present invention is not limited to any particular mechanism. Indeed, a mechanism is not needed in order to practice the present invention. Nonetheless, it is contemplated that there are at least two general mechanisms that may be exploited in the present invention. In a preferred embodiment, the binding of a target analyte to a binding ligand which is sterically accessible to a solvent transition metal complex causes one or more of the small, polar ligands on the solvent accessible transition metal complex to be replaced by one or more coordination atoms supplied by the target analyte, causing a decrease in the solvent reorganization energy for at least two reasons. First, the exchange of a small, polar ligand for a generally larger, nonpolar ligand that will generally exclude more water from the metal, lowering the required solvent reorganization energy (i.e. an inner sphere λ effect). Secondly, the proximity of a generally large target analyte to the relatively small redox active molecule will sterically exclude water within the first or second coordination sphere of the metal ion, also decreasing the solvent reorganization energy.  
      Alternatively, a preferred embodiment does not necessarily require the exchange of the polar ligands on the metal ion by a target analyte coordination atom. Rather, in this embodiment, the polar ligands are effectively irreversibly bound to the metal ion, and the decrease in solvent reorganization energy is obtained as a result of the exclusion of water in the first or second coordination sphere of the metal ion as a result of the binding of the target analyte; essentially the water is excluded (i.e. an outer sphere λ o  effect).  
      Accordingly, the present invention provides methods for the detection of target analytes. The methods generally comprise binding an analyte to a binding ligand that is either associated with (forming a redox active complex) or near to a transition metal complex. In some embodiments, the transition metal complex is bound to an electrode. In some embodiments, binding of the transition metal complex to an oligo is accomplished through the use of a conductive oligomer. Thus, in some embodiments, upon analyte binding, the reorganization energy of the transition metal complex decreases to form a solvent inhibited transition metal complex, to allow greater electron transfer between the solvent inhibited transition metal complex and the electrode.  
      Accordingly, the present invention provides methods for the detection of target analytes. By “target analyte” or “analyte” or grammatical equivalents herein is meant any molecule, compound or particle to be detected. As outlined below, target analytes preferably bind to binding ligands, as is more fully described below.  
      Suitable analytes include organic and inorganic molecules, including biomolecules. In a preferred embodiment, the analyte may be an environmental pollutant (including pesticides, insecticides, toxins, etc.); a chemical (including solvents, polymers, organic materials, etc.); therapeutic molecules (including therapeutic and abused drugs, antibiotics, oligonucleotides, etc.); biomolecules (including hormones, cytokines, proteins, lipids, carbohydrates, cellular membrane antigens and receptors (neural, hormonal, nutrient, and cell surface receptors) or their ligands, etc); whole cells (including procaryotic (such as pathogenic bacteria) and eucaryotic cells, including mammalian tumor cells); viruses (includin etroviruses, herpesviruses, adenoviruses, lentiviruses, etc.); and spores; etc. Particularly preferred analytes are environmental pollutants; nucleic acids; proteins (including enzymes, antibodies, antigens, growth factors, cytokines, etc); therapeutic and abused drugs; cells; and viruses.  
      By “nucleic acid” or “oligonucleotide” or grammatical equivalents herein means at least two nucleotides covalently linked together. A nucleic acid of the present invention will generally contain phosphodiester bonds, although in some cases, as outlined below, a nucleic acid analogs are included that may have alternate backbones, comprising, for example, phosphoramide (Beaucage et al., Tetrahedron 49(10):1925 (1993) and references therein; Letsinger, J. Org. Chem. 35:3800 (1970); Sprinzl et al., Eur. J. Biochem. 81:579 (1977); Letsinger et al., Nucl. Acids Res. 14:3487 (1986); Sawai et al, Chem. Lett. 805 (1984), Letsinger et al., J. Am. Chem. Soc. 110:4470 (1988); and Pauwels et al., Chemica Scripta 26:141 91986)), phosphorothioate, phosphorodithioate, O-methylphophoroamidite linkages (see Eckstein, Oligonucleotides and Analogues: A Practical Approach, Oxford University Press), and peptide nucleic acid backbones and linkages (see Egholm, J. Am. Chem Soc. 114:1895 (1992); Meier et al., Chem. Int. Ed. Engl. 31:1008 (1992); Nielsen, Nature, 365:566 (1993); Carlsson et al., Nature 380:207 (1996), all of which are incorporated by reference). Nucleic acids containing one or more carbocyclic sugars are also included within the definition of nucleic acids (See, e.g., Jenkins et al., Chem. Soc. Rev. (1995) pp169-176). These modifications of the ribose-phosphate backbone may be done to facilitate the addition of moieties, or to increase the stability and half-life of such molecules in physiological environments.  
      The nucleic acids may be single stranded or double stranded, as specified, or contain portions of both double stranded or single stranded sequence. The nucleic acid may be DNA, both genomic and cDNA, RNA or a hybrid, where the nucleic acid contains any combination of deoxyribo- and ribo-nucleotides, and any combination of bases, including uracil, adenine, thymine, cytosine, guanine, inosine, xathanine and hypoxathanine, etc. As used herein, the term “nucleoside” includes nucleotides, and modified nucleosides such as amino or thio modified nucleosides.  
      By “proteins” or grammatical equivalents herein is meant proteins, oligopeptides and peptides, and analogs, including proteins containing non-naturally occuring amino acids and amino acid analogs, and peptidomimetic structures.  
      As will be appreciated by those in the art, a large number of analytes may be detected using the present compositions and methods; basically, any target analyte for which a binding ligand may be detected using the methods of the invention.  
      In a preferred embodiment, the target analyte is added or introduced to a redox active complex. In some embodiments, the redox active complex is attached to an electrode. As used herein, the term “redox active complex” refers to a complex comprising at least one transition metal complex and at least one binding ligand, which, as more fully described below, may be associated in a number of different ways (See, e.g., Examples 1, 3 and 4). By “transition metal complex” or “redox active molecule” or “electron transfer moiety” herein is meant a metal-containing compound which is capable of reversibly or semi-reversibly transfering one or more electrons. It is to be understood that electron donor and acceptor capabilities are relative; that is, a molecule which can lose an electron under certain experimental conditions will be able to accept an electron under different experimental conditions. It is to be understood that the number of possible transition metal complexes is very large, and that one skilled in the art of electron transfer compounds will be able to utilize a number of compounds in the present invention. Transition metals are those whose atoms have a partial or complete d shell of electrons. Suitable transition metals for use in the invention include, but are not limited to, cadmium (Cd), copper (Cu), cobalt (Co), palladium (Pd), zinc (Zn), iron (Fe), ruthenium (Ru), rhodium (Rh), osmium (Os), rhenium (Re), platinium (Pt), scandium (Sc), titanium (Ti), Vanadium (V), chromium (Cr), manganese (Mn), nickel (Ni), Molybdenum (Mo), technetium (Tc), tungsten (W), and iridium (Ir). In some embodiments, the first series of transition metals, the platinum metals (Ru, Rh, Pd, Os, Ir and Pt), along with Fe, Re, W, Mo and Tc, are preferred. Particularly preferred are metals that do not change the number of coordination sites upon a change in oxidation state, including ruthenium, osmium, iron, platinium and palladium, with ruthenium and iron being especially preferred. Generally, transition metals are depicted herein as M.  
      The transition metal ions are complexed with ligands that serve to provide the coordination atoms for the binding of the metal ion. Generally, it is the composition or characteristics of the ligands that determine whether a transition metal complex is solvent accessible. By “solvent accessible transition metal complex” or grammatical equivalents herein is meant a transition metal complex that has at least one, preferably two, and more preferably three, four or more small polar ligands. The actual number of polar ligands will depend on the coordination number (n) of the metal ion. Preferred numbers of polar ligands are (n−1) and (n−2). For example, for hexacoordinate metals, such as Fe, Ru, and Os, solvent accessible transition metal complexes preferably have one to five small polar ligands, with two to five being preferred, and three to five being particularly preferred, depending on the requirement for the other sites, as is more fully described below. Tetracoordinate metals such as Pt and Pd preferably have one, two or three small polar ligands.  
      It should be understood that “solvent accessible and solvent inhibited” are relative terms. That is, at high applied energy, even a solvent accessible transition metal complex may be induced to transfer an electron. As generally used herein, a solvent accessible transition metal complex has a first redox potential that is higher than the second redox potential of the solvent inhibited transition metal complex. In some cases, the first redox potential is so high that the voltage required will destroy or degrade the capture ligands, binding ligands and/or target analytes.  
      In some embodiments, the other coordination sites of the metal are used for attachment of the transition metal complex to either a binding ligand (directly or indirectly using a linker), to form a redox active complex, or to an electrode (frequently using a spacer, as is more fully described below), or both. Thus for example, when the transition metal complex is directly joined to a binding ligand (e.g., biotin or avidin), one, two or more of the coordination sites of the metal ion may be occupied by coordination atoms supplied by the binding ligand (or by the linker, if indirectly joined) (See, e.g., Examples 2-4, and  FIGS. 1 and 2 ). In addition, or alternatively, one or more of the coordination sites of the metal ion may be occupied by a spacer used to attach the transition metal complex to the electrode. For example, when the transition metal complex is attached to the electrode separately from the binding ligand, all of the coordination sites of the metal (n) except 1 (n−1) may contain polar ligands.  
      Suitable small polar ligands, generally depicted herein as “L”, fall into two general categories. In one embodiment, the small polar ligands will be effectively irreversibly bound to the metal ion, due to their characteristics as generally poor leaving groups or as good sigma donors, and the identity of the metal. These ligands may be referred to as “substitutionally inert”. Alternatively, as is more fully described below, the small polar ligands may be reversibly bound to the metal ion, such that upon binding of a target analyte, the analyte may provide one or more coordination atoms for the metal, effectively replacing the small polar ligands, due to their good leaving group properties or poor sigma donor properties. These ligands may be referred to as “substitutionally labile”. The ligands preferably form dipoles, since this will contribute to a high solvent reorganization energy.  
      Irreversible ligand groups include, but are not limited to, amines (—NH 2 , —NHR, and NR 2 , with R being a substitution group that is preferably small and hydrophilic, as will be appreciated by those in the art), cyano groups (—CN), thiocyano groups (—SCN), and isothiocyano groups (—NCS). Reversible ligand groups include, but are not limited to, H 2 O and halide atoms or groups. It should be understood that the change in solvent reorganization energy is quite high when a water molecule serves as a coordination atom; thus, the replacement or addition of a single water molecule on a redox active molecule will generally result in a detectable change, even when the other ligands are not small polar ligands. Thus, in a preferred embodiment, the invention relies on the replacement or addition of at least one water molecule on a redox active molecule.  
      In addition to small polar ligands, the metal ions may have additional, hydrophobic ligands, also depicted herein as “L”. That is, a hexacoordinate metal ion such as Fe may have one ligand position (preferably axial) filled by the spacer used for attachment to the electrode, two ligand positions filled by phenanthroline, and two or three small polar ligands, depending on the linkage to the binding ligand. As appreciated by those in the art, a wide variety of suitable ligands may be used. Suitable traditional ligands include, but are not limited to, pyridine, isonicotinamide; imidazole; bipyridine and substituted derivatives of bipyridine; terpyridine and substituted derivatives; phenanthrolines, particularly 1,10-phenanthroline (abbreviated phen) and substituted derivatives of phenanthrolines such as 4,7-dimethylphenanthroline and dipyridol[3,2-a:2′,3′-c]phenazine (abbreviated dppz); dipyridophenazine; 1,4,5,8,9,12-hexaazatriphenylene (abbreviated hat); 9,10-phenanthrenequinone diimine (abbreviated phi); 1,4,5,8-tetraazaphenanthrene (abbreviated tap); 1,4,8,11-tetra-azacyclotetradecane (abbreviated cyclam), isocyanide and metallocene ligands. Substituted derivatives, including fused derivatives, may also be used.  
      The presence of at least one small, polar ligand on the transition metal complex makes the solvent reorganization energy high, which suppresses electron transfer to and from the transition metal redox active molecule. Thus, in some embodiments, a solvent accessible redox active molecule has a solvent reorganization energy of greater than about 500 meV, with greater than about 800 meV being preferred, greater than about 1 eV being preferred and greater than about 1.2 to 1.3 eV being particularly preferred.  
      In addition to the solvent accessible redox active molecule, a redox active complex comprises a binding ligand which will bind the target analyte (e.g., avidin or biotin). By “binding ligand” or grammatical equivalents herein is meant a compound that is used to probe for the presence of the target analyte, and that will specifically bind to the analyte; the binding ligand is part of a binding pair. By “specifically bind” herein is meant that the ligand binds the analyte, with specificity sufficient to differentiate between the analyte and other components or contaminants of the test sample. This binding should be sufficient to remain bound under the conditions of the assay, including wash steps to remove non-specific binding. Generally, the disassociation constants of the analyte to the binding ligand will be in the range of at least 10 −4  to 10 −6  M −1 , with a preferred range being 10 −5  to 10 −9  M −1  and a particularly preferred range being 10 −7  to 10 −4 M −4 .  
      As will be appreciated by those in the art, the composition of the binding ligand will depend on the composition of the target analyte. Binding ligands to a wide variety of analytes are known or can be readily found using known techniques. For example, when the analyte is a single-stranded nucleic acid, the binding ligand may be a complementary nucleic acid, or when the analyte is avidin, the binding ligand may be biotin. Alternatively, the binding ligand may be a nucleic acid-binding protein when the analyte is a single or double-stranded nucleic acid. When the analyte is a protein (e.g., a hormone), the binding ligands include proteins or small molecules (e.g., a hormone receptor). Preferred binding ligand proteins include peptides. For example, when the analyte is an enzyme, suitable binding ligands include substrates and inhibitors. Antigen-antibody pairs, receptor-ligands, and carbohydrates and their binding partners are also suitable analyte-binding ligand pairs.  
      Together, the transition metal complex and the binding ligand comprise a redox active complex. In addition, there may be more than one binding ligand (e.g., the same or different binding ligand) or transition metal complex per redox active complex. The redox active complex may also contain additional moieties, such as cross-linking agents, labels, etc., and linkers for attachment to an electrode.  
      In some embodiments, the redox active complex is bound to an electrode. This may be accomplished in any number of ways, as will be apparent to those in the art. Generally, as is more fully described below, one or both of the transition metal complex and the binding ligand are attached, via a spacer, to the electrode.  
      In some embodiments, the redox active complex is covalently attached to the electrode via a spacer. By “spacer” herein is meant a moiety which holds the redox active complex off the surface of the electrode. In a preferred embodiment, the spacer is a conductive oligomer as described herein, although suitable spacer moieties include passivation agents and insulators as outlined below. The spacer moieties may be substantially non-conductive, although preferably (but not required) is that the electron coupling between the redox active molecule and the electrode (HAB) does not become the rate limiting step in electron transfer.  
      In general, the length of the spacer is as described for conductive polymers and passivation agents. As will be appreciated by those in the art, if the spacer becomes too long, the electronic coupling between the redox active molecule and the electrode will decrease.  
      In a preferred embodiment, the spacer is a conductive oligomer. By “conductive oligomer” herein is meant a substantially conducting oligomer, preferably linear, some embodiments of which are referred to in the literature as “molecular wires”. Conductive oligomers, and their synthesis, use and attachment to moieties is described in PCT US97/20014, hereby expressly incorporated in its entirety.  
      By “substantially conducting” herein is meant that the electron coupling between the transition metal complex and the electrode (HAB) through the oligomer is not the rate limiting step of electron transfer. Generally, the conductive oligomer has substantially overlapping π-orbitals, i.e. conjugated 7α-orbitals, as between the monomeric units of the conductive oligomer, although the conductive oligomer may also contain one or more sigma (σ) bonds. Additionally, a conductive oligomer may be defined functionally by its ability to pass electrons into or from an attached transition metal complex. Furthermore, the conductive oligomer is more conductive than the insulators as defined herein.  
      In a preferred embodiment, the conductive oligomers have a conductivity, S, of from between about 10 −6  to about 10 −4  Ω −1  cm −1 , with from about 10 −5  to about 10 −3  Ω −1  cm −1  being preferred, with these S values being calculated for molecules ranging from about 20 to about 200. As described below, insulators have a conductivity S of about 10 −7  Ω −1  cm −1  or lower, with less than about 10 −8  Ω −1  cm −1  being preferred. (See generally Gardner et al., Sensors and Actuators A 51 (1995) 57-66, incorporated herein by reference).  
      Desired characteristics of a conductive oligomer include high conductivity, sufficient solubility in organic solvents and/or water for synthesis and use of the compositions of the invention, and preferably chemical resistance to reactions that occur i) during synthesis of the redox active complexes, ii) during the attachment of the conductive oligomer to an electrode, or iii) during analyte assays.  
      In some embodiments, the oligomers of the invention comprise at least two monomeric subunits. As is described more fully below, oligomers include homo- and hetero-oligomers, and include polymers.  
      In a preferred embodiment, the conductive oligomer has the structure depicted Structure 1:1 of U.S. Pat. App. No 20020033345, herein incorporated by reference in its entirety for all purposes.  
      As will be understood by those in the art, all of the structures depicted herein may have additional atoms or structures (e.g., the oligo of Structure 1 mentioned above may be attached to transition metal complexes or redox active complexes, binding ligands, electrodes, etc. or to several of these). In some embodiments, the conductive oligomers are attached at the left side to an electrode; that is, as depicted in Structure 1, the left “Y” is connected to the electrode and the right “Y”, if present, is attached to the redox active complex, i.e. either the transition metal complex or binding ligand, either directly or through the use of a linker, as is described in U.S. Pat. App. No 20020033345.  
      By “aromatic group” or grammatical equivalents herein is meant an aromatic monocyclic or polycyclic hydrocarbon moiety generally containing 5 to 14 carbon atoms (although larger polycyclic rings structures may be made) and any carbocylic ketone or thioketone derivative thereof, wherein the carbon atom with the free valence is a member of an aromatic ring. Aromatic groups include arylene groups and aromatic groups with more than two atoms removed. For the purposes of this application aromatic includes heterocycle. “Heterocycle” or “heteroaryl” means an aromatic group wherein 1 to 5 of the indicated carbon atoms are replaced by a heteroatom chosen from nitrogen, oxygen, sulfur, phosphorus, boron and silicon wherein the atom with the free valence is a member of an aromatic ring, and any heterocyclic ketone and thioketone derivative thereof. Thus, heterocycle includes thienyl, furyl, pyrrolyl, pyrimidinyl, oxalyl, indolyl, purinyl, quinolyl, isoquinolyl, thiazolyl, imidozyl, etc.  
      In some embodiments, the Y aromatic groups of a conductive oligomer may be different, i.e. the conductive oligomer may be a heterooligomer. That is, a conductive oligomer may comprise an oligomer of a single type of Y groups, or of multiple types of Y groups. Thus, in a preferred embodiment, when a barrier monolayer is used as is described below, one or more types of Y groups are used in the conductive oligomer within the monolayer with a second type(s) of Y group used above the monolayer level. Thus, the conductive oligomer may comprise Y groups that have good packing efficiency within the monolayer at the electrode surface, and a second type(s) of Y groups with greater flexibility and hydrophilicity above the monolayer level to facilitate target analyte binding. For example, unsubstituted benzyl rings may comprise the Y rings for monolayer packing, and substituted benzyl rings may be used above the monolayer.  
      Alternatively, heterocylic rings, either substituted or unsubstituted, may be used above the monolayer. Additionally, in one embodiment, heterooligomers are used even when the conductive oligomer does not extend out of the monolayer.  
      The aromatic group may be substituted with a substitution group, generally depicted herein as R. R groups may be added as necessary to affect the packing of the conductive oligomers, i.e. when the conductive oligomers form a monolayer on the electrode, R groups may be used to alter the association of the oligomers in the monolayer. R groups may also be added to 1) alter the solubility of the oligomer or of compositions containing the oligomers; 2) alter the conjugation or electrochemical potential of the system; and 3) alter the charge or characteristics at the surface of the monolayer.  
      Suitable R groups include, but are not limited to, hydrogen, alkyl, alcohol, aromatic, amino, amido, nitro, ethers, esters, aldehydes, ketones, iminos, sulfonyl, silicon moieties, halogens, sulfur containing moieties, phosphorus containing moieties, and ethylene glycols. In the structures depicted herein, P is hydrogen when the position is unsubstituted. It should be noted that some positions may allow two substitution groups, R and R′, in which case the R and R′ groups may be either the same or different.  
      By “alkyl group” or grammatical equivalents herein is meant a straight or branched chain alkyl group, with straight chain alkyl groups being preferred. If branched, it may be branched at one or more positions, and unless specified, at any position. The alkyl group may range from about 1 to about 30 carbon atoms (C1-C30), with a preferred embodiment utilizing from about 1 to about 20 carbon atoms (C1-C20), with about C1 through about C12 to about C15 being preferred, and C1 to C5 being particularly preferred, although in some embodiments the alkyl group may be much larger. Also included within the definition of an alkyl group are cycloalkyl groups such as C5 and C6 rings, and heterocyclic rings with nitrogen, oxygen, sulfur, silicon or phosphorus. Alkyl also includes heteroalkyl, with heteroatoms of sulfur, oxygen, nitrogen, and silicone being preferred. Alkyl includes substituted alkyl groups. By “substituted alkyl group” herein is meant an alkyl group further comprising one or more substitution moieties “R”, as defined above.  
      By “amino groups” or grammatical equivalents herein is meant —NH 2 , —NHR and —NR 2  groups, with R being as defined herein.  
      By “nitro group” herein is meant an —NO 2  group.  
      By “sulfur containing moieties” herein is meant compounds containing sulfur atoms, including but not limited to, thia-, thio- and sulfo-compounds, thiols (—SH and —SR), sulfides (—RSR—), sulfoxides (—R—SO—R—), sulfones (—R—SO 2 —R—), disulfides (—R—S—S—R—) and sulfonyl ester (R—SO 2 —O—R) groups. By “phosphorus containing moieties” herein is meant compounds containing phosphorus, including, but not limited to, phosphines and phosphates. By “silicon containing moieties” herein is meant compounds containing silicon, including siloxanes.  
      By “ether” herein is meant an —O—R group.  
      By “ester” herein is meant a —COOR group; esters include thioesters (—CSOR).  
      By “halogen” herein is meant bromine, iodine, chlorine, or fluorine. Preferred substituted alkyls are partially or fully halogenated alkyls such as CF 3 , etc.  
      By “aldehyde” herein is meant —RCOH groups.  
      By “ketone” herein is meant —R—CO—R groups.  
      By “alcohol” herein is meant —OH groups, and alkyl alcohols —ROH.  
      By “amido” herein is meant —RCONH— or RCONR— groups.  
      By “imino” herein is meant and —R—CNH—R— and —R—CNR—R— groups.  
      By “ethylene glycol” herein is meant a —(O—CH 2 —CH 2 ) n — group, although each carbon atom of the ethylene group may also be singly or doubly substituted, i.e. —(O—CR 2 —CR 2 ) n —, with R as described above. Ethylene glycol derivatives with other heteroatoms in place of oxygen (i.e. —(N—CH 2 —CH 2 ) n — or —(S—CH 2 —CH 2 ) n —, or with substitution groups) are also preferred.  
      Preferred substitution groups include, but are not limited to, methyl, ethyl, propyl, and ethylene glycol and derivatives thereof.  
      Preferred aromatic groups include, but are not limited to, phenyl, naphthyl, naphthalene, anthracene, phenanthroline, pyrole, pyridine, thiophene, porphyrins, and substituted derivatives of each of these, included fused ring derivatives.  
      As will be appreciated by those in the art, a large number of possible conductive oligomers may be utilized. These include conductive oligomers, as are generally known in the art, including for example, compounds comprising fused aromatic rings or Teflon™-like oligomers, such as —(CF 2 ) n —, —(CHF) n — and —(CFR) n — (See for example, Schumm et al., Angew. Chem. Intl. Ed. Engl. 33:1361 (1994); Grosshenny et al., Platinum Metals Rev. 40(1):26-35 (1996); Tour, Chem. Rev. 96:537-553 (1996); Hsung et al., Organometallics 14:48084815 (1995; and references cited therein, all of which are expressly incorporated by reference).  
      In some embodiments, the present invention provides compositions comprising binding metal complexes. In some embodiments, the compex comprises [(4-BMP)Ru(NH 3 ) 5 ] 2+ , [(4-DMP)Ru(NH 3 ) 5 ] 2+ , [(4-DPEP)Ru(NH 3 ) 5 ] 2+ , or [(Bbpy)Fe(CN) 4 ] 2−  (See, e.g., Examples 1-5). In some embodiments, the complex further comprises biotin.  
      In an alternative embodiment, the binding metal complex may comprise a mixture of different types of metal complexes, for example of [(4-DMP)Ru(NH 3 ) 5 ] 2+  and [(4-DPEP)Ru(NH 3 ) 5 ] 2+ .  
      The conductive oligomers are covalently attached to the redox active complexes, transition metal complexes (collectively redox active moieties), or binding ligands. By “covalently attached” herein is meant that two moieties are attached by at least one bond, including sigma bonds, pi bonds and coordination bonds.  
      The attachment of the metal ion is generally done by attaching a substitutionally inert ligand to the end of the spacer. In a preferred embodiment, this ligand is monodentate, or at most bidentate, although other polydentate ligands may also be used. Thus, for example, an amino or imidazole group (monodentate) or a phenathroline (bidentate) may be attached to the end of the spacer using techniques well known in the art, or techniques outlined in PCT US97/20014, hereby expressly incorporated by reference.  
      The attachment of the binding ligand to either the metal ion or the spacer is also done using well known techniques, and will depend on the composition of the binding ligand. When the binding ligand is a nucleic acid, either double-stranded or single-stranded, attachment to the metal ion can be done as is described in PCT US97/20014.  
      In general, attachment of the binding ligand to either the metal ion or the spacer is done using functional groups either naturally found on the binding ligand or added using well known techniques. These groups can be at the terminus of the binding ligand, for example at the N- or C-terminus of a protein, or at any internal position. Thus, amino, thio, carboxyl or amido groups can all be used for attachment. Similarly, chemical attachment of traditional ligands such as pyridine or phenanthroline may also be done, as will be appreciated by those in the art. For example, attachment of proteinaceous binding ligands is generally done using functional groups present on the amino acid side chains or at the N- or C-terminus; for example, any groups such as the N-terminus or side chains such as histidine may serve as ligands for the metal ion. Similarly, attachment of carbohydrate binding ligands is generally done by derivatizing the sugar to serve as a metal ion ligand. Alternatively, these groups may be used to attach to the spacer, using well known techniques. In any of these embodiments, there may be additional connector or linkers present. For example, when the binding ligand is a proteinaceous enzyme substrate or inhibitor, there may be additional amino acids, or an alkyl group, etc., between the metal ion ligand and the functional substrate or inhibitor.  
      In addition, as noted herein, two or more binding ligands may be attached to a single redox active complex. For example, in some embodiments, two single-stranded nucleic acids may be attached, such that the binding of a complementary target sequence will change the solvent reorganization energy of the redox active molecule. In this embodiment, the two single stranded nucleic acids are designed to allow for a “gap” in the complementary sequence to accomodate the metal ion; this is generally from 1 to 3 nucleotides.  
      In some embodiment, the binding ligand and the redox active molecule do not form a redox active complex, but rather are each individually attached to the electrode, generally via a spacer. In this embodiment, it is the proximity of the redox active molecule to the target analyte bound to the binding ligand that results in a decrease of the solvent reorganization energy upon binding. Preferably, the solvent accessible redox active molecule is within 12 angstroms of some portion of the target analyte, with less than about 8 angstroms being preferred and less than about 5 angstroms being particularly preferred, and less than about 3.5 angstroms being especially preferred. It should be noted that the distance between the binding ligand and the redox active molecule may be much larger, depending on the size of the target analyte. Thus, the binding of a large target analyte may reduce the solvent reorganization energy of a solvent accessible redox active molecule many angstroms away from the binding ligand.  
      In some embodiments, a single binding event of a target analyte to a binding ligand can result in a decrease in solvent reorganization energy for a number of transition metal complexes, if the density of the transition metal complexes is high enough in the area of the binding ligand, or the target analyte is large enough. Similarly, different binding ligands for the same target analyte may be used; for example, to “tack down” a large target analyte on the surface, to effect as many transition metal complexes as possible per single target analyte.  
      In some embodiments, the redox moieties and binding ligands are attached to an electrode, via a spacer as outlined above. Thus, one end or terminus of the conductive oligomer is attached to the redox moiety or binding ligand, and the other is attached to an electrode. In some embodiments it may be desirable to have the conductive oligomer attached at a position other than a terminus, or to have a branched conductive oligomer that is attached to an electrode at one terminus and to a redox active molecule and a binding ligand at other termini. Similarly, the conductive oligomer may be attached at two sites to the electrode.  
      By “electrode” herein is meant a composition, which, when connected to an electronic device, is able to sense a current or charge and convert it to a signal. Preferred electodes are known in the art and include, but are not limited to, certain metals and their oxides, including gold; platinum; palladium; silicon; aluminum; metal oxide electrodes including platinum oxide, titanium oxide, tin oxide, indium tin oxide, palladium oxide, silicon oxide, aluminum oxide, molybdenum oxide (Mo 2 O 6 ), tungsten oxide (WO 3 ) and ruthenium oxides; and carbon (including glassy carbon electrodes, graphite and carbon paste). Preferred electrodes include gold, silicon, carbon and metal oxide electrodes.  
      The electrodes described herein are depicted as a flat surface, which is only one of the possible conformations of the electrode and is for schematic purposes only. The conformation of the electrode will vary with the detection method used. For example, flat planar electrodes may be preferred for optical detection methods, or when arrays are made, thus requiring addressable locations for both synthesis and detection. Alternatively, for single analyte analysis, the electrode may be in the form of a tube, with the compositions of the invention bound to the inner surface. This allows a maximum of surface area containing the binding ligand to be exposed to a small volume of sample.  
      The covalent attachment of the conductive oligomer containing the redox active moieties and binding ligands of the invention may be accomplished in a variety of ways, depending on the electrode and the conductive oligomer used. In some embodiments, some type of linker is used.  
      In a preferred embodiment, the electrode is a gold electrode, and attachment is via a sulfur linkage as is well known in the art. Although the exact characteristics of the gold-sulfur attachment are not known, this linkage is considered covalent for the purposes of this invention.  
      In a preferred embodiment, the electrode is a carbon electrode, i.e. a glassy carbon electrode, and attachment is via a nitrogen of an amine group.  
      In general, one of two general schemes may be followed to synthesize the compositions of the invention. In a preferred embodiment, the spacer is synthesized and the redox active complex, comprising the redox active molecule and the binding ligand is also made separately. These two are added together, and then added to the electrode. Alternatively, in a preferred embodiment, the spacer is made and attached to the electrode. The redox active complex is made, and then it is added to the spacer. General synthetic schemes may be found in PCT US97/20014.  
      Thus, in a preferred embodiment, electrodes are made that comprise conductive oligomers attached to redox active moieties and/or binding ligands for the purposes of analyte assays, as is more fully described herein. As will be appreciated by those in the art, electrodes can be made that have a single species of binding ligand (i.e. specific for a particular analyte) or multiple binding ligand species (i.e. specific for two or more analytes).  
      In addition, as outlined herein, the use of a solid support such as an electrode enables the use of these binding ligands in an array form. The use of arrays of binding ligands specific for oligonucleotides are well known in the art. In addition, techniques are known for “addressing” locations within an electrode and for the surface modification of electrodes.  
      Thus, in a preferred embodiment, arrays of different binding ligands are laid down on the electrode, each of which are covalently attached to the electrode via a conductive linker. In this embodiment, the number of different species of binding ligands may vary widely, from one to thousands, with from about 4 to about 100,000 being preferred, and from about 10 to about 10,000 being particularly preferred.  
      In a preferred embodiment, the electrode further comprises a passivation agent, preferably in the form of a monolayer on the electrode surface. For some analytes, such as nucleic acids, the efficiency of analyte binding (i.e. hybridization) may increase when the binding ligand is at a distance from the electrode. In addition, the presence of a monolayer can decrease non-specific binding to the surface. A passivation agent layer facilitates the maintenance of the binding ligand and/or analyte away from the electrode surface. In addition, a passivation agent serves to keep charge carriers away from the surface of the electrode. Thus, this layer helps to prevent electrical contact between the electrodes and the electron transfer moieties, or between the electrode and charged species within the solvent. Such contact can result in a direct “short circuit” or an indirect short circuit via charged species which may be present in the sample. Accordingly, the monolayer of passivation agents is preferably tightly packed in a uniform layer on the electrode surface, such that a minimum of “holes” exist. Alternatively, the passivation agent may not be in the form of a monolayer, but may be present to help the packing of the conductive oligomers or other characteristics.  
      The passivation agents thus serve as a physical barrier to block solvent accesibility to the electrode. As such, the passivation agents themselves may in fact be either (1) conducting or (2) nonconducting, i.e. insulating, molecules. Thus, in one embodiment, the passivation agents are conductive oligomers, as described herein, with or without a terminal group to block or decrease the transfer of charge to the electrode. Other passivation agents which may be conductive include oligomers of —(CF 2 ) n —, —(CHF) n — and —(CFR) n —. In a preferred embodiment, the passivation agents are insulator moieties.  
      An “insulator” is a substantially nonconducting oligomer, preferably linear. By “substantially nonconducting” herein is meant that the rate of electron transfer through the insulator is slower than the rate of electron transfer through the a conductive oligomer. Stated differently, the electrical resistance of the insulator is higher than the electrical resistance of the conductive oligomer. It should be noted however that even oligomers generally considered to be insulators, such as —(CH 2 ) 16  molecules, still may transfer electrons, albeit at a slow rate.  
      In a preferred embodiment, the insulators have a conductivity, S, of about 10 −7  o −7  or lower, with less than about  10   −8  o −1  cm −1  being preferred. (See generally Gardner et al., supra).  
      Generally, insulators are alkyl or heteroalkyl oligomers or moieties with σ bonds, although any particular insulator molecule may contain aromatic groups or one or more conjugated bonds. By “heteroalkyl” herein is meant an alkyl group that has at least one heteroatom, e.g., nitrogen, oxygen, sulfur, phosphorus, silicon or boron included in the chain. Alternatively, the insulator may be quite similar to a conductive oligomer with the addition of one or more heteroatoms or bonds that serve to inhibit or slow, preferably substantially, electron transfer.  
      The passivation agents, including insulators, may be substituted with R groups as defined herein to alter the packing of the moieties or conductive oligomers on an electrode, the hydrophilicity or hydrophobicity of the insulator, and the flexibility, e.g., the rotational, torsional or longitudinal flexibility of the insulator. For example, branched alkyl groups may be used. In addition, the terminus of the passivation agent, including insulators, may contain an additional group to influence the exposed surface of the monolayer. For example, the addition of charged, neutral or hydrophobic groups may be done to inhibit non-specific binding from the sample, or to influence the kinetics of binding of the analyte, etc. For example, there may be negatively charged groups on the terminus to form a charged surface such that when the nucleic acid is DNA or RNA the nucleic acid is repelled or prevented from lying down on the surface.  
      The length of the passivation agent will vary as needed. Generally, the length of the passivation agents is similar to the length of the conductive oligomers, as outlined above. In addition, the conductive oligomers may be basically the same length as the passivation agents or longer than them, resulting in the binding ligands being more accessible to the solvent.  
      The monolayer may comprise a single type of passivation agent, including insulators, or different types.  
      Suitable insulators are known in the art, and include, but are not limited to, —(CH 2 ) n —, —(CRH) n —, and —(CR 2 ) n —, ethylene glycol or derivatives using other heteroatoms in place of oxygen, e.g., nitrogen or sulfur (sulfur derivatives are not preferred when the electrode is gold).  
      The passivation agents are generally attached to the electrode in the same manner as the conductive oligomer, and may use the same linker as defined above.  
      In some embodiments, the target analyte, contained within a test sample, is added to the electrode containing either a solvent accessible redox active complex (e.g., comprising an avidin-binding metal complex) or a mixture of solvent accessible transition metal complexes and binding ligands, under conditions that if present, the target analyte will bind to the binding ligand. These conditions are generally physiological conditions. Generally a plurality of assay mixtures are run in parallel with different concentrations to obtain a differential response to the various concentrations. Typically, one of these concentrations serves as a negative control, i.e., at zero concentration or below the level of detection. In addition, any variety of other reagents may be included in the screening assay. These include reagents like salts, neutral proteins, e.g. albumin, detergents, etc which may be used to facilitate optimal binding and/or reduce non-specific or background interactions. Also reagents that otherwise improve the efficiency of the assay, such as protease inhibitors, nuclease inhibitors, anti-microbial agents, etc., may be used. The mixture of components may be added in any order that provides for the requisite binding.  
      In some embodiments, the assay system is a solution based assay. For example, in some embodiments, a solvent accessible transition metal complex is covalently attached to a binding ligand by a linker. In this context, a number of linkers may be used. In general, linkers are selected to facilitate both binding of the target analyte to the binding ligand, e.g. the linker is used to avoid steric hinderance of binding. For example, as shown in the Examples, the use of the sulfur atom of biotin as a coordination atom of the transition metal complex hinders the binding of biotin to avidin. It should be noted that this type of linkage, e.g. “direct” linkage of the binding ligand and the transition metal complex, will be appropriate in other systems, for example for proteins whose binding pocket requirements are not as rigid as those of the biotin/avidin system. Thus for example the use of one or more amines in a peptide as coordination moieties for a transitional metal complex for the binding of a protease or other protein is suitable for use in the invention.  
      In addition to controlling binding ligand/target analyte binding, linkers also find use in controlling the optimal spacing between the binding ligand and the transition metal complex such that binding can occur and there is a change in redox potential.  
      In some embodiments, linkers (e.g., for use in solution phase assays) include, but are not limited to —(CR 2 ) n —, wherein n is an integer from 1-4, more preferably from 4 to 15, and even more preferably from 4 to 10, wherein R is independently selected from the substitutents outlined above but is preferably hydrogen. Also preferred are heteroalkyl from 4 to 15 atoms, again which may be optionally substituted at any position, or saturated. In addition, preferred linkers terminate in a ligand as outlined above, to attach the transition metal complex. A particular embodiment comprises a saturated alkyl group, again from C4 to C15 with C4 to C10 being preferred, terminating in a pyridine (optionally substituted at any position).  
      In some embodiments, the target analyte will bind the binding ligand reversibly, i.e. non-covalently, such as in protein-protein interactions of antigens-antibodies, enzyme-substrate (or some inhibitors) or receptor-ligand interactions.  
      In some embodiments, the target analyte will bind the binding ligand irreversibly, for example covalently. For example, some enzyme-inhibitor interactions are considered irreversible. Alternatively, the analyte initially binds reversibly, with subsequent manipulation of the system which results in covalent attachment. For example, chemical cross-linking after binding may be done, as will be appreciated by those in the art. For example, peptides may be cross-linked using a variety of bifunctional agents, such as maleimidobenzoic acid, methyldithioacetic acid, mercaptobenzoic acid, S-pyridyl dithiopropionate, etc. Alternatively, functionally reactive groups on the target analyte and the binding ligand may be induced to form covalent attachments.  
      Upon binding of the analyte to the binding moiety, the solvent accessible transition metal complex becomes solvent inhibited. By “solvent inhibited transition metal complex” herein is meant the solvent reorganization energy of the solvent inhibited transition metal complex is less than the solvent reorganization energy of the solvent accessible transition metal complex. As noted above, this may occur in several ways. In a preferred embodiment, the target analyte provides a coordination atom, such that the solvent accessible transition metal complex loses at least one, and preferably several, of its small polar ligands. Alternatively, in a preferred embodiment, the proximity of the target analyte to the transition metal complex does not result in ligand exchange, but rather excludes solvent from the area surrounding the metal ion (i.e. the first or second coordination sphere) thus effectively lowering the required solvent reorganization energy.  
      In a preferred embodiment, the required solvent reorganization energy decreases sufficiently to result in a decrease in the E 0  of the redox active molecule by at about 100 mV, with at least about 200 mV being preferred, and at least about 300-500 mV being particularly preferred (See, e.g., Examples 3-5,  FIGS. 5-9 ).  
      In a preferred embodiment, the required solvent reorganization energy decreases by at least 100 mV, with at least about 200 mV being preferred, and at least about 300-500 mV being particularly preferred.  
      In a preferred embodiment, the required solvent reorganization energy decreases sufficiently to result in a rate change of electron transfer (kET) between the solvent inhibited transition metal complex and the electrode relative to the rate of electron transfer between the solvent accessible transition metal complex and the electrode. In a preferred embodiment, this rate change is greater than about a factor of 3, with at least about a factor of 10 being preferred and at least about a factor of 100 or more being particularly preferred.  
      The determination of solvent reorganization energy will be done as is appreciated by those in the art. Briefly, as outlined in Marcus theory, the electron transfer rates (kET) are determined at a number of different driving forces (or free energy) the point at which the rate equals the free energy is the activationless rate (A). This may be treated in most cases as the equivalent of the solvent reorganization energy; (See, e.g., Gray et al. Ann. Rev. Biochem. 65:537 (1996), hereby incorporated by reference).  
      The solvent inhibited transition metal complex, indicating the presence of a target analyte, is detected by intiating electron transfer and detecting a signal characteristic of electron transfer between the solvent inhibited redox active molecule and the electrode.  
      In some embodiments, electron transfer is initiated electronically, with voltage being preferred. A potential is applied to a sample containing modified nucleic acid probes. Precise control and variations in the applied potential can be via a potentiostat and either a three electrode system (one reference, one sample and one counter electrode) or a two electrode system (one sample and one counter electrode). This allows matching of applied potential to peak electron transfer potential of the system which depends in part on the choice of transition metal complexes and in part on the conductive oligomer used.  
      In preferred embodiments, initiation and detection is chosen to maximize the relative difference between the solvent reorganization energies of the solvent accessible and solvent inhibited transition metal complexes.  
      It is contemplated that electron transfer between the transition metal complex and the electrode can be detected in a variety of ways, with electronic detection, including, but not limited to, amperommetry, voltammetry, capacitance and impedance being preferred. These methods include time or frequency dependent methods based on AC or DC currents, pulsed methods, lock-in techniques, and filtering (high pass, low pass, band pass). In some embodiments, all that is required is electron transfer detection; in others, the rate of electron transfer may be determined.  
      In a preferred embodiment, electronic detection is used, including amperommetry, voltammetry, capacitance, and impedance. Suitable techniques include, but are not limited to, electrogravimetry; coulometry (including controlled potential coulometry and constant current coulometry); voltametry (cyclic voltametry, pulse voltametry (normal pulse voltametry, square wave voltametry, differential pulse voltametry, Osteryoung square wave voltametry, and coulostatic pulse techniques); stripping analysis (aniodic stripping analysis, cathiodic stripping analysis, square wave stripping voltammetry); conductance measurements (electrolytic conductance, direct analysis); time-dependent electrochemical analyses (chronoamperometry, chronopotentiometry, cyclic chronopotentiometry and amperometry, AC polography, chronogalvametry, and chronocoulometry); AC impedance measurement, capacitance measurement; AC voltametry, and photoelectrochemistry.  
      In a preferred embodiment, monitoring electron transfer is via amperometric detection. This method of detection involves applying a potential (as compared to a separate reference electrode) between the electrode containing the compositions of the invention and an auxiliary (counter) electrode in the test sample. Electron transfer of differing efficiencies is induced in samples in the presence or absence of target analyte.  
      The device for measuring electron transfer amperometrically involves sensitive current detection and includes a means of controlling the voltage potential, usually a potentiostat. This voltage is optimized with reference to the potential of the redox active molecule.  
      In a preferred embodiment, alternative electron detection modes are utilized. For example, potentiometric (or voltammetric) measurements involve non-faradaic (no net current flow) processes and are utilized traditionally in pH and other ion detectors. Similar sensors are used to monitor electron transfer between the redox active molecules and the electrode. In addition, other properties of insulators (such as resistance) and of conductors (such as conductivity, impedance and capicitance) could be used to monitor electron transfer between the redox active molecules and the electrode. Finally, any system that generates a current (such as electron transfer) also generates a small magnetic field, which may be monitored in some embodiments.  
      In a preferred embodiment, the system may be calibrated to determine the amount of solvent accessible transition metal complexes on an electrode by running the system in organic solvent prior to the addition of target. This is quite significant to serve as an internal control of the sensor or system. This allows a preliminary measurement, prior to the addition of target, on the same molecules that will be used for detection, rather than rely on a similar but different control system. Thus, the actual molecules that will be used for the detection can be quantified prior to any experiment. Running the system in the absence of water, i.e. in organic solvent such as acetonitrile, will exclude the water and substantially negate any solvent reorganization effects. This will allow a quantification of the actual number of molecules that are on the surface of the electrode. The sample can then be added, an output signal determined, and the ratio of bound/unbound molecules determined. This is a significant advantage over prior methods.  
      It should be understood that one benefit of the fast rates of electron transfer observed in the compositions of the invention is that time resolution can greatly enhance the signal-to-noise results of monitors based on electronic current The fast rates of electron transfer of the present invention result both in high signals and stereotyped delays between electron transfer initiation and completion. By amplifying signals of particular delays, such as through the use of pulsed initiation of electron transfer and “lock-in” amplifiers of detection, orders of magnitude improvements in signal-to-noise may be achieved.  
      Without being bound by theory, it appears that target analytes, bound to an electrode, may respond in a manner similar to a resistor and capacitor in series. Also, the E 0  of the redox active molecule can shift as a result of the target analyte binding. Furthermore, it may be possible to distinguish between solvent accessible and solvent inhibited transition metal complexes on the basis of the rate of electron transfer, which in turn can be exploited in a number of ways for detection of the target analyte. Thus, as will be appreciated by those in the art, any number of initiation-detection systems can be used in the present invention.  
      In some embodiments, electron transfer is initiated and detected using direct current (DC) techniques. As noted above, the E 0  of the redox active molecule can shift as a result of the change in the solvent reorganization energy upon target analyte binding. Thus, measurements taken at the E 0  of the solvent accessible transition metal complex and at the E 0  of the solvent inhibited complex will allow the detection of the analyte. As will be appreciated by those in the art, a number of suitable methods may be used to detect the electron transfer.  
      In some embodiments, electron transfer is initiated using alternating current (AC) methods. A first input electrical signal is applied to the system, preferably via at least the sample electrode (containing the complexes of the invention) and the counter electrode, to initiate electron transfer between the electrode and the second electron transfer moiety. Three electrode systems may also be used, with the voltage applied to the reference and working electrodes. In this embodiment, the first input signal comprises at least an AC component. The AC component may be of variable amplitude and frequency. Generally, for use in the present methods, the AC amplitude ranges from about 1 mV to about 1.1 V, with from about 10 mV to about 800 mV being preferred, and from about 10 mV to about 500 mV being especially preferred. The AC frequency ranges from about 0.01 Hz to about 10 MHz, with from about 1 Hz to about 1 MHz being preferred, and from about 1 Hz to about 100 kHz being especially preferred In some embodiments, the first input signal comprises a DC component and an AC component. That is, a DC offset voltage between the sample and counter electrodes is swept through the electrochemical potential of the electron transfer moiety. The sweep is used to identify the DC voltage at which the maximum response of the system is seen. This is generally at or about the electrochemical potential of the transition metal complex. Once this voltage is determined, either a sweep or one or more uniform DC offset voltages may be used. DC offset voltages of from about −1 V to about +1.1 V are preferred, with from about −500 mV to about +800 mV being especially preferred, and from about −300 mV to about 500 mV being particularly preferred. On top of the DC offset voltage, an AC signal component of variable amplitude and frequency is applied. If the transition metal complex has a low enough solvent reorganization energy to respond to the AC perturbation, an AC current will be produced due to electron transfer between the electrode and the transition metal complex.  
      In some embodiments, the AC amplitude is varied. Without being bound by theory, it appears that increasing the amplitude increases the driving force. Thus, higher amplitudes, which result in higher overpotentials give faster rates of electron transfer. Thus, generally, the same system gives an improved response (i.e. higher output signals) at any single frequency through the use of higher overpotentials at that frequency. Thus, the amplitude may be increased at high frequencies to increase the rate of electron transfer through the system, resulting in greater sensitivity. In addition, as noted above, it may be possible to distinguish between solvent accessible and solvent inhibited transition metal complexes on the basis of the rate of electron transfer, which in turn can be used either to distinguish the two on the basis of frequency or overpotential.  
      In some embodiments, measurements of the system are taken at at least two separate amplitudes or overpotentials, with measurements at a plurality of amplitudes being preferred. As noted above, changes in response as a result of changes in amplitude may form the basis of identification, calibration and quantification of the system.  
      In some embodiments, the AC frequency is varied. At different frequencies, different molecules respond in different ways. As will be appreciated by those in the art, increasing the frequency generally increases the output current. However, when the frequency is greater than the rate at which electrons may travel between the electrode and the transition metal complexes, higher frequencies result in a loss or decrease of output signal. At some point, the frequency will be greater than the rate of electron transfer through even solvent inhibited transition metal complexes, and then the output signal will also drop.  
      In addition, the use of AC techniques allows the significant reduction of background signals at any single frequency due to entities other than the target analyte, i.e. “locking out” or “filtering” unwanted signals. That is, the frequency response of a charge carrier or redox active species in solution will be limited by its diffusion coefficient. Accordingly, at high frequencies, a charge carrier may not diffuse rapidly enough to transfer its charge to the electrode, and/or the charge transfer kinetics may not be fast enough. This is particularly significant in embodiments that do not utilize a passivation layer monolayer or have partial or insufficient monolayers, i.e. where the solvent is accessible to the electrode. As outlined above, in DC techniques, the presence of “holes” where the electrode is accessible to the solvent can result in solvent charge carriers “short circuiting” the system. However, using the present AC techniques, one or more frequencies can be chosen that prevent a frequency response of one or more charge carriers in solution, whether or not a monolayer is present. This is particularly significant since many biological fluids such as blood contain significant amounts of redox active species which can interfere with amperometric detection methods.  
      In some embodiments, measurements of the system are taken at at least two separate frequencies, with measurements at a plurality of frequencies being preferred. A plurality of frequencies includes a scan. In a preferred embodiment, the frequency response is determined at at least two, preferably at least about five, and more preferably at least about ten frequencies.  
      After transmitting the input signal to initiate electron transfer, an output signal is received or detected. The presence and magnitude of the output signal will depend on the overpotential/amplitude of the input signal; the frequency of the input AC signal; the composition of the intervening medium, i.e. the impedance, between the electron transfer moieties; the DC offset; the environment of the system; and the solvent. At a given input signal, the presence and magnitude of the output signal will depend in general on the solvent reorganization energy required to bring about a change in the oxidation state of the metal ion. Thus, upon transmitting the input signal, comprising an AC component and a DC offset, electrons are transferred between the electrode and the transition metal complex, when the solvent reorganization energy is low enough, the frequency is in range, and the amplitude is sufficient, resulting in an output signal.  
      In some embodiments, the output signal comprises an AC current. As outlined above, the magnitude of the output current will depend on a number of parameters. By varying these parameters, the system may be optimized in a number of ways.  
      In general, AC currents generated in the present invention range from about 1 femptoamp to about 1 milliamp, with currents from about 50 femptoamps to about 100 microamps being preferred, and from about 1 picoamp to about 1 microamp being especially preferred.  
      In addition, those in the art will appreciate that it is also possible to use the compositions of the invention in assays that rely on a loss of signal. For example, a first measurement is taken when the transition metal complex is inhibited, and then the system is changed as a result of the introduction of a target analyte, causing the solvent inhibited molecule to become solvent accessible, resulting in a loss of signal. This may be done in several ways, as will be appreciated by those in the art.  
      In some embodiments, a first measurement is taken when the target analyte is present. The target analyte is then removed, for example by the use of high salt concentrations or thermal conditions, and then a second measurement is taken. The quantification of the loss of the signal can serve as the basis of the assay.  
      Alternatively, the target analyte may be an enzyme. In this preferred embodiment, the transition metal complex is made solvent inhibited by the presence of an enzyme substrate or analog, preferably, but not required to be covalently attached to the transition metal complex, preferably as one or more ligands. Upon introduction of the target enzyme, the enzyme associates with the substrate to cleave or otherwise sterically alter the substrate such that the transition metal complex is made solvent accessible. This change can then be detected. This embodiment is advantageous in that it results in an amplification of the signal, since a single enzyme molecule can result in multiple solvent accessible molecules. This may find particular use in the detection of bacteria or other pathogens that secrete enzymes, particularly scavenger proteases or carbohydrases.  
      Similarly, a preferred embodiment utilizes competition-type assays. In this embodiment, the binding ligand is the same as the actual molecule for which detection is desired; that is, the binding ligand is actually the target analyte or an analog. A binding partner of the binding ligand is added to the surface, such that the transition metal complex becomes solvent inhibited, electron transfer occurs and a signal is generated. Then the actual test sample, containing the same or similar target analyte which is bound to the electrode, is added. The test sample analyte will compete for the binding partner, causing the loss of the binding partner on the surface and a resulting decrease in the signal.  
      A similar embodiment utilizes a target analyte (or analog) is covalently attached to a preferably larger moiety (a “blocking moiety”). The analyte-blocking moiety complex is bound to a binding ligand that binds the target analyte, serving to render the transition metal complex solvent inhibited. The introduction of the test sample target analyte serves to compete for the analyte-blocking moiety complex, releasing the larger complex and resulting in a more solvent accessible molecule.  
      In addition, while the majority of the above discussion is directed to the use of the invention when the compositions are attached to surfaces such as electrodes, those of skill in the art will appreciate that solution-based systems are also possible. In this embodiment, solvent accessible transition metal complexes are attached to binding ligands (either directly or using short linkers that keep the binding ligand and the transition metal complex in close enough proximity to allow detection) to form soluble redox active complexes. Upon binding of an analyte, the transition metal complex becomes solvent inhibited, and a change in the system can be detected. In a preferred embodiment, the reaction is monitored by fluorescence or electrochemical means. Alternatively, the reaction may be monitored electronically, using mediators.  
      The present invention further provides apparatus for the detection of analytes using AC detection methods. The apparatus includes a test chamber which has at least a first measuring or sample electrode, and a second measuring or counter electrode. Three electrode systems are also useful. The first and second measuring electrodes are in contact with a test sample receiving region, such that in the presence of a liquid test sample, the two electrodes may be in in electrical contact.  
      In a preferred embodiment, the first measuring electrode comprises a redox active complex, covalently attached via a spacer, and preferably via a conductive oligomer, such as are described herein. Alternatively, the first measuring electrode comprises covalently attached transition metal complexes and binding ligands.  
      The apparatus further comprises a voltage source electrically connected to the test chamber; that is, to the measuring electrodes. Preferably, the voltage source is capable of delivering AC and DC voltages, if needed.  
      In a preferred embodiment, the apparatus further comprises a processor capable of comparing the input signal and the output signal. The processor is coupled to the electrodes and configured to receive an output signal, and thus detect the presence of the target analyte. The compositions of the present invention may be used in a variety of research, clinical, quality control, or field testing settings.  
     EXPERIMENTAL  
      The following examples are provided in order to demonstrate and further illustrate certain preferred embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof.  
      In the experimental disclosure which follows, the following abbreviations apply: ° C. (degrees Centigrade); cm (centimeters); g (grams); l or L (liters); μg (micrograms); μl (microliters); μm (micrometers); μM (micromolar); μmol (micromoles); mg (milligrams); ml (milliliters); mm (millimeters); mM (millimolar); mmol (millimoles); M (molar); mol (moles); ng (nanograms); nm (nanometers); nmol (nanomoles); N (normal); and pmol (picomoles).  
     Example 1  
     Materials and Methods  
      Materials.  
      4-aminomethylpyridine (95% from Aldrich) was distilled before use and stored at 4° C. under argon. [Ru(NH 3 ) 6 ]Cl 3  (Strem) and [Ru(NH 3 ) 5 Cl]Cl 2  (98% from Strem) were recrystallized from 0.1N HCl before usage. ZnHg amalgam was prepared and stored under argon for no more than one month before usage. Egg white avidin (Molecular Probes), 4,4′-bipyridine (Aldrich), methylviologen (Aldrich), TSTU (Aldrich), NH 4  PF 6  (Strem), biotin (Aldrich), HABA-avidin test reagent (Sigma) and desthiobiotin (Sigma) were used as received. Buffers were stored at 4° C. for no more than one month before usage.  
      Synthesis.  
      All operations involving Ru were carried out under argon according to standard Schlenk techniques. [(H 2 O)Ru(NH 3 ) 5 ] 2+  was either generated and isolated as the PF 6   −  salt according to the literature procedure and then used or the or Cl −  salt was used directly (See, e.g., Callahan et al., Inorg. Chem. 1975, 14, 1443-1453). Electrochemical experiments were carried out using a CH Instruments 660A workstation, a glassy carbon working electrode, a Ag/AgCl reference electrode and a platinum wire counter electrode.  
      5-(2-Oxo-hexahydro-thieno[3,4-d]imidazol-4-yl)-pentanoic acid (pyridin-4-ylmethyl)-amide (4-BMP). A procedure according to Bannwarth and Knorr was followed with the following modifications (See, e.g., Bannwarth and Knorr, R, Tetrahedron Lett. 1991, 32, 1157-1160). Once the final reaction was done as monitored by TLC, a scoop of silica was added and the solvent removed. The column was loaded with the dry silica. Elution of the compound was accomplished with 8:1 CHCl 3 :MeOH. The product as collected from this column was not pure, but could be further purified by running a second column using straight MeOH followed by recrystallization from MeOH and diethyl ether.  
      6-(5-Methyl-2-oxo-imidazolidin-4-yl)-hexanoic acid (pyridin-4-ylmethyl)-amide (4-DMP). A procedure according to Bannwarth and Knorr was followed with the following modifications (See, e.g., Bannwarth and Knorr, R, Tetrahedron Lett. 1991, 32, 1157-1160). A solution of 0.30 g (1.4 mmol) desthiobiotin in 3.75 mL DMF was prepared and to this was added 0.507 g (1.68 mmol) TSTU and several drops of Et 3 N with stirring. After 1 h, 0.165 mL (0.174 g, 1.61 mmol) 4-aminomethylpyridine was added. The mixture was stirred for 18 h during which time it turned dark brown. The solvent was removed, the residue redissolved in 11:1 CHCl 3 :MeOH and loaded onto a silica gel column and eluted with the same mixture. The product was collected as a colorless oil which was recrystallized from CHCl 3  and diethyl ether. Yield: 90%.  
      6-(5-Methyl-2-oxo-imidazolidin-4-yl)-hexanoic acid 5-pyridin-4-yl-pentyl ester (4-DPEP). 5-Pyridin-4-yl-pentan-1-ol was synthesized according to the literature (See, e.g., Iglesias et al., Tetrahedron 2001, 57, 3125-3130). A 0.10 (0.60 mmol) portion of this alcohol was combined with 0.129 g (0.6 mmol) desthiobiotin, 0.137 g DCC, 0.081 g DMAP in 5 mL CH 2 Cl 2  and stirred overnight. The product was purified by silica gel chromatography (8.33:1 CHCl 3 :MeOH). [(4-BMP)Ru(NH 3 ) 5 ][PF 6 ] 2  A suspension of [Ru(NH 3 ) 5 Cl]Cl 2  in H 2 O was prepared and several pieces of ZnHg amalgam were added. The mixture was stirred for 30-45 min and filtered into a flask containing 4-BMP. The solution turned dark yellow quickly. After stirring 1 h, the solution was transferred to a flask containing NH 4 PF 6  in 1 mL H 2 O. A yellow precipitate formed immediately which was filtered and washed sequentially with cold water, ethanol and diethyl ether. The yellow powder was dried in vacuo. Yield: 46%. UV-visible (solvent) λ max =414 nm.  
      [(4-DMP)RU(NH 3 ) 5 ][PF 6 ] 2 . A suspension of 0.03 g [Ru(NH 3 ) 5 Cl]Cl 2  in H 2 O was prepared and several pieces of ZnHg amalgam were added. The mixture was stirred for 30-45 min and filtered into a flask containing 75 mg of 4-DMP. The solution turned dark yellow quickly. After stirring 1 h, the solution was transferred to a flask containing 0.6 g of NH 4 PF 6  in 0.6 mL H 2 O. A yellow precipitate formed immediately which was filtered and washed sequentially with cold water, and diethyl ether. The yellow powder was dried in vacuo. Yield: 67%.  
      [(4-DPEP)Ru(NH 3 ) 5 ][PF 6 ] 2 . A solution of 0.10 g (0.277 mmol) DPEP and 0.026 g (0.053 mmol) [(H 2 O)Ru(NH 3 ) 5 ](PF 6 ) 2  in 6 mL acetone was stirred for 30 min. The solution turned dark yellow quickly. The solvent was removed in vacuo and the yellow residue recrystallized from CHCl3 and diethyl ether. The yellow powder was dried in vacuo. [(B-bpy)Fe(CN) 4 ] 2−  Was synthesized as outline in  FIG. 4 .  
      Electrochemistry.  
      A solution of [(4-BMP)Ru(NH 3 ) 5 ](PF 6 ) 2  in pH 9.3 phosphate buffer was prepared and the cyclic voltammogram obtained. Two reversible redox events (based on i a /i p  and ΔE p ) were observed at +363 and +93 mV (vs Ag.AgCl). The current showed normal dependence on scan rate.  
      A solution of 1.0 mg [(4-DMP)Ru(NH 3 ) 5 ](PF 6 ) 2  in 4.2 mL 0.5M NaCl in H 2 O was prepared and the cyclic voltammogram obtained. A reversible redox event at was observed at +79 mV. To this was added 26 mg avidin, the CV obtained, and then 0.25 mL of a solution of methyl viologen (10 mg/5 mL; 7.77×10 −3 M) was added and the CV remeasured.  
      To test biotin displacement of the (4-DMP) complex, a solution of 0.80 mg [(4-DMP)Ru(NH 3 ) 5 ](PF 6 ) 2  in 16.5 mL pH 7.1 HEPES buffer was prepared and the electrochemistry recorded. A 19 mg portion of avidin was added, the CV and square wave voltammograms were recorded. Upon addition of a 25-fold excess of (4.8 mg) biotin the signal was regenerated.  
      A 4.8×10 −5  M solution (0.75 mg) of [(4-DPEP)Ru(NH 3 ) 5 ](PF 6 ) 2  in pH 7.18 phosphate buffer was prepared and the cyclic voltammogram obtained. A reversible redox event was observed at +77 mV that disappeared upon addition of avidin.  
     Example 2  
     Synthesis of [(4-BMP)Ru(NH 3 ) 5 ] 2+   
      In some embodiments, the system utilizes avidin, a protein that is highly stable over wide ranges of temperature and pH and is resistant to denaturation, and binds to a ligand, biotin with the highest affinity of all known non-covalent protein-small molecule interactions (k d =10 −15 ) (See, e.g., Green Biochem. J. 1963, 89, 609-620; Green, Biochem. J. 1963, 89, 599-609). Further, electrochemical experiments involving both immobilized and solubilized avidin have shown that the protein does not lose biotin-binding capability over the duration of the electrochemical experiments (See, e.g., Anzai, Chem. Lett. 1993, 1231-1234; Masarik et al., Anal. Chem. 2003, 75, 2663-2669; Sugawara et al., Anal. Chem. 1995, 67, 299-302; Sugawara et al., Bioelectroch. Bioener. 1996, 39, 309-312; Padeste et al., Biosens. Bioelectron. 2003, 19, 239-247). Biotin is commercially available and has a carboxylic acid functional group to facilitate covalent conjugation to the redox center.  
      Biotin was conjugated to 4-aminomethylpyridine following a procedure modified from that previously reported in which N,N,N′,N′-tetramethyl(succinimido)uronium tetrafluoroborate (TSTU) is used to activate the carboxylic acid by generating the succinimidyl ester (Bannwarth et al., Tetrahedron Lett. 1991, 32, 1157-1160). Addition of the amine leads to rapid formation of the amide bond. After two column chromatographic purification steps and recrystallization from MeOH and diethyl ether the compound was isolated in good yield. The avidin-binding compound desthiobiotin was conjugated to 4-aminomethylpyridine and purified following a similar procedure to that of 4-BMP. An extended-chain avidin-binding ligand was also prepared. First, 5-pyridin-4-yl-pentan-1-ol was prepared according to previsouly reported method (Iglesias et al., Tetrahedron 2001, 57, 3125-3130). The alcohol was then coupled to desthiobiotin using DCC/DMAP in CH 2 Cl 2  to give the ester 4-DPEP (See, e.g.,  FIG. 1 ). Qualitatively, using the commercially available HABA-avidin test reagent, these ligands were found to bind avidin at a rate comparable to that of biotin and desthiobiotin. All ligands were characterized by  1 H,  13 C NMR spectroscopy and ESI-MS.  
      An excess of 4-BMP was combined with either [(H 2 O)Ru(NH 3 ) 5 ](PF 6 ) 2  in acetone or with freshly generated [(H 2 O)Ru(NH 3 ) 5 ](Cl) 2  in H 2 O and stirred under argon for 1 h (See, e.g.,  FIG. 2 ). In the former case, the acetone was removed in vacuo and the yellow residue recrystallized multiple times from MeOH. In the aqueous case, an excess of NH 4  PF 6  was added to give a yellow precipitate which was filtered, washed with water and ethanol and dried to give [(4-BMP) N/S Ru(NH 3 ) 5 ](PF 6 ) 2 . The UV-visible spectrum of the mixture showed absorbances at 214, 250, 260 (shoulder), and 414 nm in pH 8.0 phosphate buffer. A mixture of two compounds was detected in the cyclic voltammogram (pH 9.3 phosphate buffer) at +363 and +93 mV (+560 and +290 mV vs. N.H.E.). These results are consistent with the presence of an S-bound and N-bound linkage isomers.  
      As a comparison, [(H 2 O)Ru(NH 3 ) 5 ] 2+  was treated with an excess of biotin in water to form a complex in which the thioether of biotin acts to bind to the Ru center. The cyclic voltammogram (CV) showed one main redox event centered at +368 mV (+565 mV vs. N.H.E.) which is very close to one of the waves observed in the [(4-BMP) N/S Ru(NH 3 ) 5 ] 2+  mixture and is also comparable to the electrochemistry of previously reported [(SMe 2 )Ru(NH 3 ) 5 ] 2+  (See, Kuehn and Taube J. Am. Chem. Soc. 1976, 98, 689-702, and  FIGS. 3 and 4 ). The formation of the S-bound complex is not surprising given the affinity of Ru(II) for thioethers (See, Kuehn and Taube J. Am. Chem. Soc. 1976, 98, 689-702). Varying the experimental conditions had little effect on the ratio of products (S- to N-bound) which was estimated from the CV to be 1:1. The N- and S-bound isomers, referred to as [(4-BMP) N Ru(NH 3 ) 5 ] 2+  and [(4-BMP) S Ru(NH 3 ) 5 ] 2+  respectively, could not be separated by recrystallization methods.  
     Example 3  
     Binding of Avidin to [(4-BMP)Ru(NH 3 ) 5 ] 2+   
      Upon treatment of the CV solution of [(4-BMP)RU(NH 3 ) 5 ] 2+  with egg white avidin, the current signal for the event centered at +93 mV, assigned to [(4-BMP) N Ru(NH 3 ) 5 ] 2+ , decreased dramatically (See, e.g.,  FIG. 5 ). In experiments conducted using the compositions and methods of the present invention, [(4-BMP) S Ru(NH 3 ) 5 ] 2+  provided an excellent internal standard as a biotin-containing complex that is not bound by avidin, and for which the redox couple is still observable by CV after the addition of avidin. Although a mechanism is not required by the present invention, and more than one method is contemplated, in some embodiments, it is contemplated that the S-bound complex [(4-BMP) S Ru(NH 3 ) 5 ] 2+  does not bind to avidin because most of the H-bonding contacts of the bound complex are at the ureido ring and adding the Ru would make this part of the molecule too large to fit in the binding pocket. Thus, because the redox event for this compound is unchanged by the addition of avidin, it is contemplated that the protein is not causing the reduction in current by fouling the electrode.  
      Examination of the circular dichroism spectrum of avidin compared to the sample used for the electrochemical experiments showed that no changes in the tertiary structure occurred due to addition of the Ru complex or due to the handling procedures (degassing and stirring) necessary for the electrochemical experiments.  
     Example 4  
     Synthesis and avidin binding of [(4-DMP)RU(NH 3 ) 5 ] 2    
      The complex [(4-DMP)Ru(NH 3 ) 5 ] 2+  was generated from [(H 2 O)Ru(NH 3 ) 5 ] 2+  and isolated similarly to the case of 4-BMP. The UV-visible spectrum was found to be consistent with an N-bound species with abosrptions at 210 and 408 nm. The  1 H NMR spectrum further confirms the N-bound state as the pyridyl proton signals are shifted with respect to free 4-DMP while the other signals remain unchanged. Electrochemistry of this compound showed a redox event at +79 mV (+276 vs. N.H.E.) assigned to the Ru(II)/(III) couple. Upon treatment of the CV solution with egg white avidin, the current signal for the event decreased. Upon addition of 25 equiv of biotin to the sample, the current signal for this couple increased nearly to the original level indicating that [(4-DMP)Ru(NH 3 ) 5 ] 2+  is displaced from avidin by biotin and is intact (See, e.g.,  FIGS. 6 and 9 ). Although the kD of desthiobtion-avidin is only slightly lower than that of biotin-avidin, it is known that biotin can displace desthiobiotin (See, e.g., Mueller et al., Science 1993, 262, 1706-1708).  
     Example 5  
     Synthesis and Avidin Binding of [(4-DPEP)Ru(NH 3 ) 5 ] 2+   
      The Ru complex [(4-DPEP)Ru(NH 3 ) 5 ] 2+  was prepared by treating freshly generated [(H 2 O)Ru(NH 3 ) 5 ] 2+  with an excess of 4-DPEP in H 2 O and isolated by the addition of NH4 PF6. The complex was purified by reprecipitation from MeOH/CHCl 3 . The CV in pH 7.1 HEPES showed a RuII/III couple at +18 mV (+215 mV vs. N.H.E.). Addition of avidin decreased the signal as observed in the other cases. This result is interesting because the methylene chain linking the Ru to the binding ligand is much longer than in the previous cases, so the Ru should be well outside the protein and should thus more accessible to solvent and the electrode than in the case of avidin-bound [(4-BMP) N Ru(NH 3 ) 5 ] 2+  and [(4-DMP)Ru(NH 3 ) 5 ] 2+ .  
      In the case of [(4-DMP)Ru(NH 3 ) 5 ] 2+  the electrochemical mediators 4,4′-bipyridine and methyl viologen were added but no effect on the current signal for the Ru couple was observed. In the case of [(4-DPEP)Ru(NH 3 ) 5 ] 2+  the oxidant [Ru(NH 3 ) 6 ] 3+  was added and the decrease of the UV-visible absorption at 410 nm was monitored over time. The rate of oxidation for the avidin-bound [(4-DPEP)Ru(NH 3 ) 5 ] 2+  was found to be qualitatively slower than for free [(4-DPEP)Ru(NH 3 ) 5 ] 2+  indicating that the Ru center is somehow being blocked by the protein (See, e.g.,  FIG. 7 ).  
      The electrochemical experiments were performed in phosphate buffer at pH 9.3, 8.1, and 7.3, in 0.5M NaCl solution, and in pH 7.3 HEPES buffer. The observed potentials of the Ru complexes shifted slightly with the change in solvent/pH as expected. The addition of avidin consistently resulted in a dramatic reduction of the current of the Ru couple.  
      Here, as in all cases, the Ru compounds were qualitatively found to bind avidin using the HABA-avidin test reagent.  
      As a comparison the biotin-containing complex [(B-bpy)Fe(CN) 4 ] 2−  was synthesized. This complex has not only a different charge than the Ru probes, but also is a useful comparison of a complex containing high-field rather than low field ligands. The Fe complex was characterized using NMR and IR. The complex was qualitatively found to bind avidin using the HABA-avidin test reagent. Electrochemical characterization of a 3.94×10 −4  M solution in pH7.0 phosphate buffer gave an reversible Fe(II)/)III) couple at +343 mV (+540 mV vs N.H.E.) with ΔE p =62 at 100 mV/s (See, e.g.,  FIG. 8 ). As with the Ru complexes, addition of an amount of avidin sufficient to bind all of the Fe complex (20.57 mg) resulted in total elimination of the current signal. A slight increase in current was observed upon addition of the mediator 4,4′-bipyridine.  
      All publications and patents mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described method and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention that are obvious to those skilled in the relevant fields are intended to be within the scope of the following claims.