Patent Publication Number: US-2006014005-A1

Title: Method for realizing a sensor device able to detect chemical substances and sensor device so obtained

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates, in its more general aspect, to a method for monitoring the presence of chemical substances, also called analytes, in a determined environment.  
      In particular, the present invention relates to a method for realizing a sensor device, of the type comprising, as detection element, an active film of metallic nanoparticles sensitive to the presence of the above chemical substances.  
      The present invention also relates to a sensor device obtained with the above method.  
      2. Description of the Related Art  
      In the field the need of realizing sensor devices able to detect the presence of one or more chemical substances in a determined environment is known, mainly of gaseous environmental pollutants, both of organic nature, such as polychlordibenz-dioxins, polychloro-biphenyls, aromatic compounds and condensate rings in general, and of inorganic nature, such as nitrogen and sulphur oxides.  
      The age-old problem of monitoring in an accurate way the organic compounds of the dioxin family, produced in the incinerators by the combustion of chlorinated plastic materials is particularly known.  
      In particular, there is a need for very sensitive sensor devices that are able to detect minimal amounts, of the order of the ppb, of these substances.  
      So as to satisfy this need, the interest is deeper and deeper in a technique for monitoring chemical substances by employing nanostructured materials. This interest has is due to the electronic transport properties of these materials.  
      Nanostructured materials substantially comprise a plurality of highly organized metallic nanoparticles, also called metallic nanoclusters. The word metallic nanoparticle means a particle having dimensions generally comprised between 0.1 and 100 nm, preferably in the order of 1-10 nm, and having a metallic nucleus, for example gold, platinum or palladium, which is covered by means of a shell for being stabilized.  
      The cover is obtained by means of capping agents, which usually comprise polymers able to maintain the metallic nuclei separated, or by means of passivating agents, which usually comprise organic compounds with reactive groups, such as thiols and amines.  
      Recent developments have shown the possibility of realizing sensor devices wherein the active matrix comprises a film of gold nanoparticles of the above specified type which are deposited on insulating layers, as described for example in the articles of A. W. Snow, H. Wohltjen and N. L. Jarvis: “MIME Chemical Vapor Microsensor”, 2002, NRL Review, and N. L. Jarvis, A. W. Snow, H. Wohltjen and R. R. Smardzewski: “CB Nanosensors”.  
      The developed technology is based on a morphologic alteration of the nanoparticle film which results in a variation of the conductivity thereof.  
      This technology is described for example in the U.S. Pat. No. 6,221,673. This document discloses the realization of a sensor device comprising a detection cell having an active matrix of packed nanoparticles. Each particle comprises a conductive metallic nucleus and a passivating shell.  
      The detection of the analytes is obtained by means of interaction of the passivating shell with the analyte so as to determine the alteration of the overall conductive property of the film of metallic nanoparticles.  
      In substance, the analyte, by interacting with the passivating shell, causes an increase of the distance between the metallic nanoparticles and, as a consequence, a decrease of the probability of electronic tunneling phenomena or electronic hopping, which are responsible for the conduction.  
      For realizing the detection cell, the known method comprises the preparation of an inert support, suitably provided with electrodes necessary for measuring the conductivity variation and generally realized by means of expensive techniques such as the traditional photolithography. In a second step a solution of the above describe nanoparticles is deposited, in suitable solvents, to form a film covering the electrodes.  
      The deposition of the film alternatively occurs according to two techniques.  
      A first technique provides to spray the solution of nebulized nanoparticles on the surface of the substrate, preferably pre-heated at a temperature higher than the boiling point of the solvents.  
      A second technique provides to initially apply, on the substrate surface, a solution of coupling agents, which are two-function substances, comprising a first functional group able to bind with the substrate and a second functional group able to bind with the nanoparticles. Afterwards, the substrate is dipped in the solution of nanoparticles so as to allow the bond with the solution and to obtain the desired film. This known method, although allowing an accurate analytes detection, has however known drawbacks still unsolved.  
      The main drawbacks of the known techniques for realizing the electrodes (lithography) are that they are very expensive, and that the deposition of nanoparticle solutions has such reproducibility problems as to prevent the realization of sensor devices on a large scale.  
      This drawback is even more evident if the need, in the environment monitoring field, of providing a high number of sensor devices is considered, having comparable and constant qualities in terms of sensitiveness and specificity of detection of determined polluting substances.  
     BRIEF SUMMARY OF THE INVENTION  
      One embodiment of the present invention is a method for the realization of the above-specified sensor device of chemical substances, which ensures a high process reproducibility and reliability, which has a reduced cost and which offers a drastic reduction of the number of steps necessary for the realization of the sensor device.  
      The method includes: 
          preparing an ink comprising a solution of metallic nanoparticles, and     jet printing the thus obtained ink so as to form an active film of metallic nanoparticles.        

      The detection mechanism of the active film is related to the variation of electric conductivity occurring as effect of the interaction between the metallic nanoparticles of the film and the chemical substances to be detected.  
      Further characteristics and advantages of the method and of the sensor according to the invention will be apparent from the following description of an embodiment thereof given by way of indicative and non limiting example with reference to the annexed drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      In the drawings:  
       FIG. 1  shows a block scheme of the method according to one embodiment of the present invention;  
       FIG. 2  shows a schematic view of the sensor device according to one embodiment of the present invention;  
       FIG. 3  shows a schematic view of an ink-jet printing step;  
       FIG. 4  shows a schematic view of interdigitated electrodes realized according to the method of  FIG. 1 .  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      With reference to the annexed drawings, reference number  10  globally indicates a scheme of a method according to the invention for the realization of a sensor device suitable for monitoring chemical substances.  
      The method  10  is used in the specific case for realizing a sensor device  20  of polluting gases, shown in  FIG. 2 , of the type comprising a detection cell  22  having an active film  24  of metallic nanoparticles formed on a supporting substrate  26 . The nanoparticles are able to interact with the polluting gases to determine a variation of the global electric conductivity of the film  24 .  
      In the specific case, gold is the metal of the nanoparticles, and the nanoparticles have a mean dimension of 5 nm.  
      The method  10  comprises as main steps: 
          a preparation step  12  of an ink comprising a solution of metallic nanoparticles, and a deposition step  14  of the ink obtained on the supporting substrate  26  by means of ink-jet printing so as to form the active film  24 .        

      The method also includes an electrode formation step  18  for forming electrodes as discussed below with respect to  FIG. 2  and an exposition step  19  in which the sensor device  20  is exposed to a controlled atmosphere enriched with a chemical substance to characterize the electric response to a known concentration of the substance.  
      The preparation step  12  of the ink preferably comprises a synthesis step  15  of synthesizing metallic nuclei, a passivation step  16  of passivating the metallic nuclei to obtain metallic nanoparticles, and a dissolution step  17  dissolving the nanoparticles in solvent to obtain the ink.  
      The synthesis  15  of the metallic nuclei is carried out according to consolidated synthesis techniques.  
      A first synthesis technique, so called by means of polyol process, is based on the oxidation-reduction reaction which occurs between a metallic precursor, in the specific case a gold precursor, and an alcholic reducing agent, which also plays the role of reaction solvent.  
      In the reaction environment a capping agent is also used for controlling the morphology and the dimensions of the metallic nanoclusters. As capping agent also a polymer can be used being soluble in the solvent and having a extension and a molecular weight suitable for the control of the metallic nucleus growth.  
      The most accredited reaction mechanism for the reduction of the metal (Me n+ ) to elemental metal (Me), in this case from Au 3+  to Au, is the following. 
 
CH 2 OH—CH 2 OH→CH 3 CHO+H 2 O 
 
2 CH 3 CHO→CH 3 CO—CO—CH 3 +H 2  
 
 n H 2 +2Me n+ →2Me+2nH+
 
 m Me+PVP→Me m -PVP→Me m+1 -PVP 
 
      In a preferred solution, trihydrated chloride of Au(III) is used (HAuCl 4 H 2 O) as metallic precursor, ethylene glycol (EG) as reducing agent and polyvinylpyrrolidone (PVP) as capping agent. An example of this synthesis is described as example in the article “P.-Y. Silvert, K. Tekaya-Elhsissen, Solid State Ionics, 1995, vol 82, pg. 53-60” and in “Volpe M. V., Longo A., Pasquini L., Casuscelli V., Carotenuto G., J. Mater. Science Letters 2003, vol. 22, pg. 1697-1699”.  
      An example of synthesis by a Polyole Process is reported at the end of the present description.  
      Afterwards the metallic nuclei are subjected to the above passivation step  16  to obtain metallic nanoparticles.  
      The passivation is preferably carried out by adding an organic compound having a reactive group, such as a thiol (R—SH) or an amine, to the solution of the PVP-stabilized metallic nuclei, which are previously obtained.  
      The type of passivating agent is not considered as limitative for the present invention, and any known passivating agent can be used, such as aliphatic thiols, straight- or branched-chained, substituted aliphatic thiols, aromatic thiols and the like.  
      Also dendrimer compounds can be used as passivating agents such as those described in “Nadeja Krasteva, Isabelle Besnard, Berit Guse, Roland E. Bauer, Klaus Mullen, Akio Yasuda, and Tobias Vossmeyer, Nano Letters 2002 Vol. 2, No. 5 pg. 551-555” and “Nadeja Krasteva, Berit Guse, Isabelle Besnard, Akio Yasuda and Tobias Vossmeyer, Sensors and Actuators B 92 (2003) 137-143”.  
      The passivation mechanism is the following: 
 
Me n —PVP+ m R—SH→Me n (SR) m   +m/ 2H 2 +PVP. 
 
      The obtained metallic nanoparticles have dimensions comprised between 4.5 and 10 nm and they are easily separated, by centrifugation, from the excess of thiol and of polymer, with obtainment of a stable, solid product.  
      Afterwards, the nanoparticles are dissolved in the above dissolution step  17  in a suitable solvent with obtainment of the ink. The choice of the solvent depends, as it will be seen more clearly hereafter, on the operative conditions of the ink-jet printing. Preferably, the used solvent is organic, such as for example toluene, chloroform, hexane and superior homologs and the like.  
      In the specific case, the obtained ink comprises a colloidal solution in toluene of gold nanoparticles.  
      According to a further embodiment, the method according to the invention comprises a synthesis step of the metallic nanoparticles by means of a so called two-phase system. This technique provides the use of a ionic metallic precursor, of a phase transfer agent and of a reducing agent. This synthesis technique is known for example from “Brust M., Walker M., Bethell D., Shiffrin D. J. And Whyman R., J. Chem. Soc., Chem. Commun., 1994, pg. 801-802”.  
      The strategy of this synthesis mechanism is that of making the metallic nanocluster grow so that a simultaneous fixing of self-assembled passivating agent monolayers occurs on the growing metallic nucleus.  
      To this purpose, the nanoparticles are grown in a two-phase system, and in particular two-phase oxidation-reduction reactions are carried out by using suitable oxidation-reduction reactants in each adjacent step.  
      The difference with respect to the previous synthesis is that the passivating step is simultaneous and competitive with the nanoparticle growth. Moreover, the procedure is of the “one pot” type since the two processes occur in the same reaction means.  
      In particular, in the specific case, HAuCl 4  is used as metallic precursor, an emulsion of H 2 O, toluene and passivating agent (for example dodecanthiol) as two-phase reaction means, tetraoctylammonium bromide ((C 8 H 17 ) 4 NBr or N(Oct) 4 Br) as phase transfer agent, and aqueous sodium borohydride (NaBH 4 ) as reducing agent. The phase transfer agent allows the transfer of the metallic precursor from water to toluene.  
      In detail, the synthesis/passivation step is set out in the following stages.  
      Initially the metallic ionic precursor is dissolved in water in the presence of N(Oct) 4 Br. Afterwards, the organic phase, constituted by the passivating agent in toluene, is added to the aqueous solution.  
      The addition is carried out under vigorous agitation so as to obtain an emulsion and to allow the transfer of Au(III) in toluene.  
      Afterwards, the reducing agent is added to the emulsion, which reduces the gold in the presence of the passivating agent.  
      The mechanism of the nucleation and growth of the metallic nanoparticles is the following: 
 
4Me n+   +n BH 4   − +3 n H 2 O→4Me+ n H 2 BO 3   − +4 n H + +2nH 2  
 
      The passivation mechanism is: 
 
Me m   +n (R—SH)→Me m (S—R) n  
 
      An example of synthesis by means of two-phase system is reported at the end of the present description.  
      By working on the reaction conditions, i.e., on the reaction temperature, the metal/passivating agent ratio, addition speed of the reducing aqueous solution it is possible to obtain nanoparticles comprised within the range between 1.5 and 5.2 nm as described for example by “M. J. Hostetler, J. E. Wingate, C.-J. Zhongh, J. E. Harris, R. W. Vachet, M. R. Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D. Porter, N. D. Evans, R. W. Murray, Langmuir 1998, 14, 17-30”.  
      The nanoparticle ink obtained by means of one technique or the other is now used, as above mentioned, for the realization of the active film in the above ink-jet printing step  14 .  
      The ink-jet printing, globally indicated with number  30  in  FIG. 3 , is carried out according to the now consolidated technology for the ink-jet printing of liquid substances.  
      As it is known, the ink-jet printing is based on the expulsion of drops  32  of liquid substances, through a head  34  provided with nozzle  36 , which represents the core of the tool.  
      In the specific case, for realizing the active film  24  any head for ink-jet printing can be used and the ink is loaded in a suitable cartridge of the printer, not shown in the drawings.  
      Preferably, a printer is used which works according to the “Drop on Demand Printing” (DOD) mode, which provides the emission of single drops  32  of ink.  
      Within this printer typology, a system  37  is preferably used for the expulsion of the ink, comprising a piezoelectric element  38 , which is connected to and co-operates with the head  34  by means of a membrane  39 .  
      The piezoelectric element  38  is subjected to electric pulses and expands and contracts according to the signal polarity. The volume variation determines a movement of the membrane  39  so as to induce the expulsion of the drop.  
      According to a further embodiment, the emission of the single drops is obtained by means of a heating element.  
      This latter is arranged inside the head for creating a steam bubble in the liquid able to induce the expulsion of the drop.  
      In case this technology is employed, it is necessary to ascertain that possible substances dissolved in the ink are not damaged by the heat.  
      In the specific case of the realization at issue an ink-jet printer of the commercial type can be used such as for example Epson (piezoelectric mechanism), or HP and Canon (thermal mechanism), or laboratory ink-jet printers such as those manufactured by Microfabi or by Microdop.  
      The procedure followed for the ink-jet printing according to one embodiment of the method of the present invention is that for example described in a detail in the document “Sawyer B. Fuller, Eric J. Wilhelm and Joseph M. Jacobson, Ink-jet Printed Nanoparticle Microelectrical Systems, Journal of Microelectromechanical Systems, Vol 11, N. 1, February (2002)”.  
      It goes without saying that, in the printing procedure, a technician of the field will resort to any precaution known in the field for optimizing the printing: for example the control of the liquid viscosity, of the nanoparticle concentration and of the interaction among the same in the solvent for avoiding the formation of self-assembled islands, the choice of the solvent, the choice of the printing head and the like.  
      For example, for ensuring a high quality of the self-assembled film it is suitable that the solvent evaporates in reasonable times, and as a consequence the organic solvents are preferred with respect to the aqueous ones. The evaporation of the solvent can be spontaneous or induced by means of suitable pre-heating of the substrate and possibly of the ink.  
      For controlling the liquid viscosity, it is sometimes convenient to use the above ink-jet head provided with heating element.  
      The material constituting the printing head is also important because it should ensure the absence of interaction with the ink. Preferably, in the case of the present invention, those materials having a higher chemical inertia are used, such as teflon, glass and the like.  
      Also the material of the substrate  26  should be as much inert as possible; to this purpose it is preferably realized with silica, with glass, with transparent polymer or similar materials.  
      In a preferred solution, before carrying out the printing step  14  for obtaining the active film  24 , the same ink is used for realizing metallic electrodes  40  and  41  ( FIG. 2 ), which, once they are placed in contact with the active film  24  and they are connected in a known way to a measurement apparatus  42 , they allow the detection of a variation of electric conductivity.  
      In this case, the method  10  comprises the ink-jet printing step  18  for the realization of the electrodes  40 ,  41 . To this purpose, the method also comprises a preliminary preparation step of a pattern for the realization of electrodes.  
      For maximizing the electric response of the sensor device  20  such geometries are chosen as to exalt the resistance variations during the exposition of the sensor device  20  to the chemical substances. Preferably, the method provides the use of interdigitated electrodes  40 ,  41  shown in  FIG. 4 .  
      According to a further embodiment, the method provides the use of two- and three-dimensional geometries by simply operating on a CAD design to be transferred to the printer.  
      After having introduced the ink into the cartridge, the deposition thereof follows by means of ink-jet printing according to the previously chosen pattern, and according to the previously described printing procedure.  
      Differently from the printing of the active film, for realizing the electrodes  40 ,  41  the substrate  26  is heated at a determined temperature, which, according to the material of the substrate and of the colloidal metal in solution, is comprised between 80 and 300° C.  
      The heating of the supporting substrate  26 , besides facilitating the solvent evaporation, is used to determine a fast and controlled desorption of the passivating agent which covers the single metallic nuclei.  
      An aim of the thermal treatment is thus the coalescence and sintering of the metallic clusters and the obtainment of a bulk metal having micronic dimensions suitable for use as electrode.  
      In the realization of the electrodes, also in this case, a technician of the field will resort to any precaution necessary so that the heating temperature of the substrate ensures: thermolysis of the chemical bonds between the surface metallic atoms and the passivating agent, 
          separation of the passivating agent in the gaseous form,     fusion of the metallic clusters,     maintenance of the substrate morphology and quality.        

      The realization by means of ink-jet printing and subsequent sintering of the nanoparticles also produces some electric contacts whose conductivity is about 70% of that of the massive gold, which is much higher than that necessary for the required conductivity measure.  
      It is to be also noted that the morphologic analysis by means of light microscopy of the electrodes obtained by means of ink-jet printing highlights a typical wave trend, which however does not invalidate the performances of the device but only pertains the morphological aspect of the system.  
      According to a further embodiment of the method according to the invention, the electrodes are not realized by means of ink-jet printing, but they are already pre-assembled in the supporting substrate by means of known technologies.  
      In a preferred solution, according to the substrate typology and to the compatibility with the solution, the method provides a preliminary treating step of the surface of the supporting substrate and of the electrodes.  
      During this preliminary step, the surfaces of the supporting substrate and of the electrodes are functionalized with coupling agents of the known type, which firmly secure the active film  24  to the substrate  26  and make the sensor device  20  more resistant with respect to the aggressive action of possible disturbing agents.  
      After the realization of the film  24  and of the electrodes  40 ,  41  the method  10  also preferably comprises the exposition step  19  to expose the active film  24  to a controlled atmosphere enriched with one or more chemical substances, such as organic substances, for characterizing the electric response with respect to a known concentration of the substances.  
      The conductivity variations are appreciated by means of resistance measures conducted during cyclic expositions of the sensor to the analyte, so as to obtain a strict correlation of a conductivity variation with the analyte amount.  
      From the description disclosed up to now, it is possible to appreciate the realization of a sensor device  20 , also object of the present invention, which comprises, as detection element, an active film  24  of metallic nanoparticles able to interact with the chemical substances to determine a conductivity variation of the film  24 . According to the invention, the film  24  comprises a printed ink of nanoparticles.  
      In a preferred solution, the sensor device  20  comprises electrodes  40 ,  41  placed in communication with the active film  24  and comprising a printed and sintered ink of nanoparticles.  
      The main advantage of the method  10  is that, thanks to the ink-jet printing technique, a high simplification of the sensor assembling procedure is obtained, which allows a drastic reduction both of the number of the stages necessary for the realization of the complete device, and a simplification of the traditionally used tools.  
      The ink-jet printing is in fact an already consolidated technique which combines a high realization simplicity of the active film and at the same time a remarkable printing precision and reliability of the film.  
      It follows that the method allows a massive decrease of the manufacturing costs and times without invalidating the efficiency and the performances of the sensor device.  
      In particular, when the printing with “Drop on Demand Printing” mode is used a high printing resolution is obtained, as well as a high printing control.  
      In this way it is possible to ensure an absolute printing reproducibility of the active film which allows very reliable sensor devices to be obtained and having constant quality and performance.  
      This makes it also possible an industrialization of the process and the obtainment, in such a way, of wide manufacturing volumes. In particular it is possible to automate all the realizing steps of the procedure as well as to bring improvements in terms of speed and cost reduction.  
      It is in fact appreciated from the above description that the method according to one embodiment of the invention comprises a limited number of operative steps for the preparation of the ink and for the printing.  
      A further advantage is that the ink-jet printing technique can be used also for the realization of electrodes. This allows a further reduction of the costs and times, with respect to currently used technologies.  
      Moreover, it is to be noted that the realization of the electrodes by means of ink-jet printing allows to use the same apparatuses used for the realization of the active film, this further reducing the manufacturing times and costs.  
      Sensor devices realized according to the technology described in the invention show remarkable potentialities in several fields of application.  
      A first use in fact relates to the integration of one or more sensor devices in apparatuses for the detection of environmental pollutants.  
      Another interesting field of application provides the use of the sensor device as detector for instrumental analysis techniques such as gas chromatography: this use responds to the need of availing of a detector with high specificity and sensitiveness.  
      Still the use of this sensor device can be thought for the detection of explosive gases in highly risky areas, also called “Electronic Noses”.  
      Example of Metallic Nanoparticle Synthesis by Means of Polyol Process  
      About 2.8-8.0 g of PVP are dissolved in 20 ml of EG. The solution is left under agitation at 60°. Afterwards 5 mg of HAuCl 4  are dissolved in 1 ml of EG and they are injected into the previously prepared hot solution. During the synthesis reaction the color of the solution changes from yellow to ruby red. For completing the synthesis reaction the solution is poured into 250 ml of acetone, and everything is sonicated, for removing the excess of EG. The resulting gel, constituted by Me-PVP, is then treated with an ethanolic solution of CH 3 (CH 2 ) 11 SH under magnetic stirring. The replacement of capping agent leads to chemisorption of thiol molecules, for carrying out the passivation. After about 1 h the passivation reaction is completed and the metallic nanoparticles passivated with thiol are separated by centrifugation from the excess of thiol and from the PVP.  
      The nanoparticles are then dispersed in a suitable solvent, such as for example toluene, for obtaining the ink.  
      Example of Synthesis by Means of Two-Phase  
      30 ml of HAuCl 4  (30 mmol dm −3 ) are mixed with a solution of N(Oct) 4 Br in toluene (80 ml, 50 mmol dm −3 ). The thus obtained two-phase mixture is vigorously mixed until AuCl 4   −  is transferred into the organic phase. At this point, 170 mg of dodecanthiol (CH 3 (CH 2 ) 11 SH) are added to the organic phase. Afterwards, 25 ml of an aqueous sodium borohydride solution (NaBH 4 ) (0.4 mol dm −3 ) are slowly added under agitation to the organic phase. After a further agitation for 3 h the organic phase is separated, evaporated up to about 10 ml and mixed with ethanol for removing the excess of thiol. The mixture is maintained for 4 h at −18° C. A dark precipitate is obtained, comprising thiol-covered gold nanoparticles, which is dissolved more times in toluene and precipitated again in ethanol.  
      The nanoparticles are then dispersed in a suitable solvent, such as for example toluene, pentane, chloroform for obtaining the desired ink.  
      By operating on the reaction conditions, i.e., on the reaction temperature, on the metal/passivating agent ratio, addition speed of the reducing aqueous solution it is possible to obtain nanoparticles comprised within the range between 1.5 and 5.2 nm as from “M. J. Hostetler, J. E. Wingate, C.-J. Zhong, J. E. Harris, R. W. Vachet, M. R. Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D. Porter, N. D. Evans, R. W. Murray, Langmuir 1998, 14, 17-30”.  
      All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheetare incorporated herein by reference, in their entirety.  
      From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.