Patent Publication Number: US-2007099238-A1

Title: Enhancement of emission using metal coated dielectric nanoparticles

Description:
BACKGROUND  
      Fluorescent dyes are used for fluorescent tagging in bioimaging and biosensor applications. Photon absorption in dyes produces a fluorescence band that is typically red-shifted from the incident photon frequency. The difference in energy between the absorbed photon and the emitted photon corresponds to the energy loss due to nonradiative processes. Metal coated spheres containing dyes have been proposed for field enhancement in Raman spectroscopy by C. Oubre and P. Nordlander in Journal of Physical Chemistry B, 108, 17740 (2004).  
     SUMMARY OF INVENTION  
      In accordance with the invention, the efficiency of fluorescent dyes or quantum dots using metal coated dielectric spheres or other metal coated dielectric shapes may be improved. Fluorescent dyes or quantum dots may be embedded in a dielectric volume of appropriate dimensions where typically half the surface of the dielectric volume is covered by a metal coating allow for increased absorption and emission efficiencies. Alternatively, fluorescent dyes or quantum dots may be attached to metal coated dielectric shapes using the appropriate chemistries.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an embodiment in accordance with the invention.  
       FIG. 2  shows the total radiated power ratio versus frequency in accordance with the invention.  
       FIG. 3  shows the total the total radiated power ratio versus frequency in accordance with the invention.  
       FIGS. 4   a - b  steps for a photochemical analytic sequence in accordance with the invention.  
       FIG. 5  shows an embodiment in accordance with the invention. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows an embodiment in accordance with the invention. Dielectric nanosphere  150  is typically fluorescent dye or quantum dot doped and shown with about half the surface of nanosphere  150  covered by metal coating  145 , typically gold or silver. Typical diameters for nanosphere  150  in accordance with the invention are typically in the range from about 0.01 μm to about 5.0 μm. Dielectric nanosphere  150  may be a nanoparticle with a non-spherical shape such as an ellipsoid or other suitable shape.  
       FIG. 2  shows the calculated ratio of total radiated power (TRP) of a single dipole inside a metal coated dielectric nanosphere versus frequency in terahertz (THz). The TRP ratio shows the ratio of the TRP for the metal coated dielectric nanosphere to the TRP of a single dipole in air. Dielectric nanosphere  150  with a refractive index of 1.5 may be fully or half metal coated and immersed in air. The fluorescent dye or quantum dot doping is approximated by single dipole  135  at the center of dielectric sphere  150 . A finite difference time domain (FDTD) is used for the calculations represented by the curves in  FIGS. 2-3 .  
      In  FIG. 2 , curves  210  and  215  correspond to dielectric nanosphere  150  having a diameter of about 25 nm with metal coating  145  having a thickness of 10 nm and 5.5 nm, respectively, and covering the entire surface of dielectric nanosphere  150 . For curves  210  and  215  the TRP ratio maximum is at about 433 and about 222 at about 749 THz and about 650 THz, respectively. In  FIG. 2 , curves  220  and  225  correspond to correspond to dielectric nanosphere  150  with metal coating  145  having a thickness of 20 nm and 11 nm, respectively, and covering half the surface of dielectric nanosphere  150 . For curves  220  and  225  the TRP ratio maximum is at about 197 and about 156 at about 634 THz and about 545 THz, respectively.  
      Metal coating  145  is a silver coating for curves  210 ,  215 ,  220  and  225 . Metal coating  145  functions as a reflector and creates a resonant cavity so that the TRP ratio is a maximum at resonance. Increasing the thickness of metal coating improves the reflective properties to increase TRP ratio while increasing the resonance frequency as seen by curves  210  and  220 . However, for thick metallic coatings  145  thicker than about 20 nm on fully metal coated dielectric nanospheres  150 , the losses in metal coating  145  typically become dominant and TRP ratio decreases. Although the above curves  210 ,  215 ,  220  and  225  were calculated for spheres, similar enhancements of TRP ratio occur for other geometrical shapes such as, for example, partially or fully metal coated cubes or cylinders.  
       FIG. 3  shows the ratio of total radiated power (TRP) of a single dipole inside a metal coated dielectric nanosphere versus frequency in terahertz (THz). Dielectric nanosphere  150  with a refractive index of 1.5 is half metal coated and immersed in air. The dye or quantum dot doping is approximated by single dipole  135  at the center of dielectric sphere  150 .  
      In  FIG. 3 , curves  310 ,  315  and  320  correspond to dielectric nanosphere  150  having diameters of about 19 nm, 25 nm and 38 nm, respectively, with metal coating  145  having a thickness of about 10 nm and being silver. For curves  310 ,  315  and  320  the TRP ratio maximum is at about 250, about 200 and about 90 at about 650 THz, about 625 THz and about 600 THz respectively. The resonance frequency and TRP ratio both decrease as the diameter of dielectric nanosphere  150  increases while keeping the thickness of metal coating  145  constant. It is expected that the resonance frequency will decrease as the diameter of nanosphere  150  increases. For a dipole on top of a flat metal plane, the TRP ratio increases by only about a factor of two for a given frequency that depends on the metal plane to dipole separation distance. Therefore, increasing the diameter of nanosphere  150  reduces the enhancement in the TRP ratio because increasing the diameter effectively increases the flatness of metal coating  145 . From  FIGS. 2 and 3  it can be concluded that the resonance frequency may be altered by changing the thickness of metal coating  145 .  
      Calculations where both the diameter of nanosphere  150  and the thickness of metal coating  145  scale accordingly indicate that the resonance frequency stays about the same. This result is expected in view of the behavior shown in  FIGS. 2 and 3  which show that the resonance frequency moves in opposite directions when only the diameter of nanosphere  150  increases and when only the thickness of metal coating  145  increases.  
       FIGS. 4   a - 4   b  show the steps for using half metal coated dielectric nanospheres  150  in a photochemical analytical sequence. Because dielectric nanospheres  150  are only half covered with metal coating  145 , selective chemistry may be performed on the surface of dielectric nanosphere  150  and on the surface of metal coating  145  in order to orient metal coating  145  to a predetermined orientation to act as a receiving cup for the incoming pump light. The relevant literature provides the specific chemistries that have been developed for attaching bio-molecules to specific surfaces such as Au, Ag, SiO 2 , polystyrene or latex. If a molecular entity that is sensitive to the specific emission wavelength of dielectric nanaosphere  150  is attached to dielectric nanosphere  150  other functions may be performed. The enhanced TRP ratio provided in accordance with the invention allows the efficient optical excitation of the molecular entity at very low concentrations which has applications in bio-sensing applications, for example.  
      Initially, functionalized surface  410  is placed in a solution containing half metal coated dielectric nanospheres  150 . The half metal coated dielectric nanospheres  150  are prepared with appropriate antibodies  415  attached to metal coating  145 . If metal coating  145  is gold, a thiol based chemistry that is complimentary to functionalized surface  410  is typically used. Typically, second set of antibodies  435  may be attached to the exposed dielectric portion of half metal coated dielectric nanospheres  150 , second set of antibodies  435  is of relevance to the subsequent chemistry that is to be performed. Half metal coated dielectric nanospheres  150  can then be positioned with metal coating  145  adjacent to the functionalized surface  410  so that the exposed dielectric portion of half metal coated dielectric nanospheres  150  is directed towards pump light  450 , attached antibodies  435  dangling radially. The procedure is performed with a variety of properly functionalized half metal coated dielectric nanospheres  150  tagged with appropriate dyes that each in turn will bind specific molecular entities  470  of interest. To perform an assay on a variety of tagged molecular entities  470 , dye or quantum dot doped half metal coated dielectric nanospheres  150  are optically pumped at an appropriate wavelength to optically excite the appropriate dye or quantum dot doped half metal coated dielectric nanosphere or nanospheres  150 . The wavelength is down shifted and re-emitted efficiently because of enhanced directionality. The efficient absorption of pump light  450  together with the highly directional and efficient re-emission of the light allows detection of molecular entities at very low concentrations. After half metal coated dielectric nanospheres  150  bind to the functionalized surface, half metal coated dielectric nanospheres  150  are typically exposed to light for photochemical analysis.  
      In addition to the geometry shown in  FIGS. 4   a - b,  an alternative geometry and chemistry in accordance with the invention involves half metal coated dielectric nanoellipsoid  550  positioned with respect to substrate  555  such that metal coating  545  is directed toward pump light  560  as shown in  FIG. 5 . The exposed dielectric portion of half metal coated dielectric nanoparticle  550  is attached to functionalized surface  555 . Work by Chew in the Journal of Chemical Physics, 87(2), 53234 (1987) and incorporated herein by reference shows that the radiative decay rate of fluorophore or quantum dot  570  placed along the axis of symmetry and a distance d typically about 2 to about 20 Angstroms from half metal coated dielectric nanoellipsoid  550  is enhanced by a factor of about 1000 for nanoellipsoid  550  with a ratio of major axis a to minor axis b having a ratio of about 1.5. This is about a factor of 10 better than for half metal coated dielectric nanosphere  150 . The surface is oriented by using the property that only about half of dielectric nanoellipsoid  550  is metal coated and fluorophore or quantum dot  570  is attached as shown in  FIG. 5 . For small dielectric nanospheres  150  or nanospheres  550 , the number of fluorophores or quantum dots  570  that can be attached is limited but because of the enhanced radiative rates there is enhanced fluorescence efficiency. The increase in radiative efficiency is about a factor of 1000 for nanoellipsoid  550  for a/b equal to about 1.5 while the improvement for nanospheres  150  in an analogous configuration is about a factor of 10.  
      Dye-doped dielectric nanospheres  150 , typically of latex or polystyrene of various sizes are commercially available, for example, from MOLECULAR PROBES CORPORATION. Synthesis of dye-doped dielectric nanoparticles of silica using a micro-emulsion method is described by S. Santra et al in Journal of Biomedical Optics 6, 160-166 (2001) and incorporated herein by reference. The silica nanoparticles can be made in uniform sizes with typical diameters from a few nanometers to a few micrometers with the size distribution controlled to within about two percent.  
      In accordance with the invention, dye-doped dielectric nanospheres  150  may be replaced with quantum dot doped dielectric nanospheres  150 . Unlike dye molecules, quantum dots absorb light at short wavelengths and the emission wavelength is determined primarily by the size of the quantum dots. This allows the absorption and emission wavelength to be further apart than for dye-doped dielectric nanospheres  150 . This provides an extra degree of freedom in addition to the thickness of metal coating  145  in designing metal coated dielectric nanospheres  150 . Because the emission and absorption processes are closely related, the absorption efficiency of the incident pump radiation may be increased resulting in an improvement in the emission efficiency.  
      Quantum dot, for example, ZnS-capped CdSe nanocrystals, doped dielectric nanospheres have been described by M. Han et al. in Nature Biotechnology, vol. 19, 631-635 (2001) and incorporated herein by reference. In particular, the dielectric material may be polystyrene or polymer. Doping by quantum dots is accomplished by swelling polystyrene nanospheres  150  in a solvent mixture containing 5 percent chloroform and 95 percent propanol or butanol and by then adding a controlled amount of ZnS-capped CdSe quantum dots. The doping process is typically complete in about 30 minutes at room temperature.  
      The first step in the metal coating process to make half metal coated doped dielectric nanospheres  150  is to disperse doped dielectric nanospheres  150  on a flat surface such as, for example, a glass slide. Polystyrene and latex nanospheres available from MOLECULAR PROBES CORPORATION are surfactant free and do not aggregate and are dispersed in a buffer fluid. Nanospheres of different sizes conjugated to biotin, avidin and streptavidin can be obtained from MOLECULAR PROBES CORPORATION. After spin coating and evaporation of the buffer fluid, the glass slide is coated with a monolayer of dielectric nanospheres  150 . The glass slide is then introduced into a sputtering system. In order to improve metal adhesion which is especially important when using gold coatings, a layer of Ti or Cr is sputter deposited to a thickness in the range from about 2 nm to about 3 nm. This is followed by a sputter deposition of metal coating  145  to the desired thickness. Because the top half of dielectric nanospheres  150  block sputter deposition of the metal on the lower half of dielectric nanospheres  150 , dielectric nanospheres  150  are typically half metal coated. Sputtering provides metal coating  145  with a high degree of metal uniformity. To achieve sufficient thickness for metal coating  145  on the sides it may be necessary to increase the metal thickness on the top. After the metal coating step, the glass slide is soaked in a suitable organic solvent, for example, propanol, to get half coated doped dielectric nanospheres  150  into solution for subsequent use. Ultra-sonification may be used if necessary to assist in removing half coated doped dielectric nanospheres  150  from the glass slide.  
      The attaching of metal specific molecules can be performed using molecules that have a terminal attaching group, for example thiol for gold, with a strong affinity to metal coating  145 ,  545  and a long linear alkyl chain that provides upright ordering on functionalized surface  410  or  555 . Affinity to the solid surface is provided by a charge-transfer complex as in alkyl thiols on noble metals as described by Porter et al. in the Journal of the American Chemical Society, 109, 3559, (1987); Bain et al. in Angewandte Chemie, International Edition, 28, 506, (1989); and Nuzzo in the Journal of the American Chemical Society, 112, 558, (1990), all incorporated herein by reference.  
      For SiO 2 , the affinity to dielectric portion of half metal coated dielectric nanospheres  150  or nanoparticle such as nanoellipsoid  550  is provided by a covalent chemical reaction, for example, silanes as described by Sagiv in the Journal of the American Chemical Society, 102, 92, 1980; Wasserman et al. in Langmuir, 5, 1074, (1989); and Ulman in Angewandte Chemie Advanced Materials, 2, 573, (1990), all incorporated by reference herein.  
      While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.