Michael Riskin

Imprinting of Molecular Recognition Sites through Electropolymerization of Functionalized Au Nanoparticles: Development of an Electrochemical TNT Sensor Based on π-Donor−Acceptor Interactions

Electrochemical sensors for the analysis of TNT with enhanced sensitivities are described. The enhanced sensitivities are achieved by tailoring pi-donor-acceptor interactions between TNT and pi-donor-modified electrodes or pi-donor-cross-linked Au nanoparticles linked to the electrode. In one configuration a p-aminothiophenolate monolayer-modified electrode leads to the analysis of TNT with a sensitivity corresponding to 17 ppb (74 nM). In the second configuration, the cross-linking of Au NPs by oligothioaniline bridges to the electrode yields a functionalized electrode that detects TNT with a sensitivity that corresponds to 460 ppt (2 nM). Most impressively, the imprinting of molecular TNT recognition sites into the pi-donor oligoaniline-cross-linked Au nanoparticles yields a functionalized electrode with a sensitivity that corresponds to 46 ppt (200 pM). The electrode reveals high selectivity, reusability, and stability.

Ultrasensitive Surface Plasmon Resonance Detection of Trinitrotoluene by a Bis-aniline-Cross-Linked Au Nanoparticles Composite

A bis-aniline-cross-linked Au nanoparticles (NPs) composite is electropolymerized on Au surfaces. The association of trinitrotoluene, TNT, to the bis-aniline bridging units via pi-donor-acceptor interactions allows the amplified detection of TNT by following the surface plasmon resonance (SPR) reflectance changes as a result of the coupling between the localized plasmon of the AuNPs and the surface plasmon wave associated with the gold surface. The detection limit for analyzing TNT by this method is approximately 10 pM. The electropolymerization of the bis-aniline-cross-linked AuNPs composite in the presence of picric acid results in a molecular-imprinted matrix for the enhanced binding of TNT. The imprinted AuNPs composite enabled the sensing of TNT with a detection limit that corresponded to 10 fM. Analysis of the SPR reflectance changes in the presence of different concentrations of TNT revealed a two-step calibration curve that included the ultrasensitive detection of TNT by the imprinted sites in the composite, KassI. for the association of TNT to the imprinted sites, 6.4 x 10(12) M-1, followed by a less sensitive detection of TNT by the nonimprinted pi-donor bis-aniline sites (KNIass. = 3.9 x 10(9) M-1). The imprinted AuNPs composite reveals impressive selectivity. The structural and functional features of the bis-aniline-cross-linked AuNPs composites were characterized by different methods including ellipsometry, AFM, and electrochemical means. The dielectric properties of the AuNPs composite in the presence of different concentrations of TNT were evaluated by the theoretical fitting of the respective experimental SPR curves. The ultrasensitive detection of the TNT by the AuNPs composite was attributed to the changes of the dielectric properties of the composite, as a result of the formation of the pi-donor-acceptor complexes between TNT and the bis-aniline units. These changes in the dielectric properties lead to a change in the conductivity of the AuNPs matrix.

Surface Plasmon Resonance Analysis of Antibiotics Using Imprinted Boronic Acid-Functionalized Au Nanoparticle Composites

Au nanoparticles (NPs) are functionalized with thioaniline electropolymerizable groups and (mercaptophenyl)boronic acid. The antibiotic substrates neomycin (NE), kanamycin (KA), and streptomycin (ST) include vicinal diol functionalities and, thus, bind to the boronic acid ligands. The electropolymerization of the functionalized Au NPs in the presence of NE, KA, or ST onto Au surfaces yields bisaniline-cross-linked Au NP composites that, after removal of the ligated antibiotics, provide molecularly imprinted matrixes which reveal high sensitivities toward the sensing of the imprinted antibiotic analytes (detection limits for analyzing NE, KA, and ST correspond to 2.00 +/- 0.21 pM, 1.00 +/- 0.10 pM, and 200 +/- 30 fM, respectively). The antibiotics are sensed by surface plasmon resonance (SPR) spectroscopy, where the coupling between the localized plasmon of the NPs and the surface plasmon wave associated with the Au surface is implemented to amplify the SPR responses. The imprinted Au NP composites are, then, used to analyze the antibiotics in milk samples.

Imprinted Au‐Nanoparticle Composites for the Ultrasensitive Surface Plasmon Resonance Detection of Hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX)

Adv. Mater. 2010, 22, 1387–1391 2010 WILEY-VCH Verlag G The analysis of explosives attracts recent research efforts due to homeland security needs and the broad demand for the clearance of minefields. While numerous studies have addressed the development of sensing platforms for nitroaromatic explosives, and specifically trinitrotoluene (TNT), the detection of more hazardous explosives, such as hexahydro-1,3,5-trinitro-1,3,5triazine (RDX) or pentaerythritol tetranitrate (PETN), is less developed and needs further efforts, particularly the improvement of the sensitivities associated with the analyses of these substrates. Different optical, electrochemical, or microgravimetric sensors or biosensors for TNTwere reported. Fluorescent organic polymers, which are quenched by nitroaromatic explosives, luminescent polysilole nanoparticles (NPs), or fluorescent silicon NPs quenched by nitroaromatic vapors enabled the development of optical sensors. The electrochemical activity of the nitro groups of TNT provided the basis for developing voltammetric sensors for this explosive, and recently, a composite of Au NPs linked to electrodes enabled a sensitive electrochemical detection of TNT. Also, different sensing matrices, such as cyclodextrin polymers, carbowax, or silicon polymers, were used for TNT analysis by surface acoustic wave devices, and the aggregation of functionalized Au NPs in the presence of TNTwas used to develop an optical sensor for the explosive. Similarly, antibody-based optical or microgravimetric quartz-crystal-microbalance biosensors for TNT were developed. Different optical or voltammetric sensors for RDX were also reported. These included the fluorescence detection of RDX with an acridinium dye, or the application of NADH-functionalized quantum dots. Also, a competitive fluorescence immunoassay for the detection of RDX was reported. The sensitivities accomplished by these methods are, however, unsatisfactory for analyzing trace amounts of the RDX explosive. Surface plasmon resonance (SPR) is a versatile method for probing changes in the refractive index occurring on thin metal films as a result of recognition events or chemical reactions. Numerous SPR sensors and biosensors were developed, and metal NPs were implemented to enhance the SPR response and to amplify SPR-based sensors. The electronic coupling between the localized plasmon of the metallic NPs (e.g., Au NPs) and the surface plasmon wave enhances the SPR response and, thus, the labeling of recognition complexes with metallic NPs amplifies the sensing events. Different biosensing processes, such as DNA hybridization, formation of immunocomplexes, and the probing of biocatalytic transformations, used Au NPs as labels for amplified SPR analyses. Recently, composites of bisaniline-crosslinked Au NPs were electropolymerized on Au electrodes, and the resulting matrices were used for the ultrasensitive SPR detection of TNT. The formation of p-donor–acceptor complexes between TNT and the p-donor bisaniline bridging units altered the dielectric properties of the Au-NP composites. This affected the coupling between the localized plasmon of the NPs and the surface plasmon wave, resulting in a shift in the surface resonance spectrum (reflectance changes), that enabled the optical readout for analyzing TNT. Theoretical modeling of the SPR shifts indicated that the charge-transfer complexes between TNT and the bisaniline bridging units altered the dielectric functions of the Au-NP composite, and this enabled the highly sensitive detection of TNT. Here, we report on the ultrasensitive SPR detection of RDX by the composites of bisaniline-crosslinked AuNPs associated with a Au surface (detection limit 12 fM). Specifically, we demonstrate that electropolymerization of the Au NPs in the presence of Kemp’s acid yields an imprinted composite with high binding affinities for RDX. This imprinting leads to the selective and sensitive detection of this explosive by SPR. Au NPs, 3.5 nm, were functionalized with a capping mixed monolayer consisting of thioaniline electropolymerizable units and mercaptoethane sulfonic acid to enhance the solubility of the NPs in an aqueous medium. The functionalized Au NPs were electropolymerized onto a thioaniline-monolayer-modified Au electrode, to yield the matrix of bisaniline-crosslinked Au NPs (Fig. 1A). Ellipsometry and coulometric analyses of the matrix of bisaniline-crosslinked Au NPs, generated by the application of ten electropolymerization cycles, indicated that the thickness of the matrix corresponded to ca. 10 nm and that ca. 4 10 bisaniline units cm 2 were electropolymerized on the electrode. Knowing the size of the Au NPs and the thickness of the composite, we estimate that approximately three random densely packed Au-NP layers compose the matrix. Complementary AFM measurements indicated that the height of the Au-NP composite is ca. 12 1 nm. The p-donor bisaniline units bridging the Au NPs could, then, associate RDX (that includes p-acceptor nitro groups), via

Optical, Electrical and Surface Plasmon Resonance Methods for Detecting Telomerase Activity

Three different sensing platforms for the analysis of telomerase activity in human cells are described. One sensing platform involves the label-free analysis of the telomerase activity by a field-effect-transistor (FET) device. The telomerase-induced extension of a primer associated with the gate of the FET device, in the presence of the nucleotide mixture dNTPs, alters the gate potential, and this allows the detection of telomerase extracted from 65 ± 10 293T (transformed human embryonic kidney) cells/μL. The second sensing platform involves the optical detection of telomerase using CdSe/ZnS quantum dots (QDs). The telomerase-stimulated telomerization of the primer-functionalized QDs in the presence of the nucleotide mixture dNTPs results in the synthesis of the G-rich telomeres. The stacking of hemin on the self-organized G-quadruplexes found on the telomers results in the electron transfer quenching of the QDs, thus providing an optical readout signal. This method enables the detection of telomerase originating from 270 ± 20 293T cells/μL. The third sensing method involves the amplified surface plasmon resonance (SPR) detection of telomerase activity. The telomerization of a primer associated with Au film-coated glass slides, in the presence of telomerase and the nucleotide mixture (dNTPs), results in the formation of telomeres on the surface, and these alter the dielectric properties of the surface resulting in a shift in the SPR spectrum. The hybridization of Au NPs functionalized with nucleic acids complementary to the telomere repeat units with the telomeres amplifies the SPR shifts due to the coupling between the local plasmon of the NPs and the surface plasmon wave. This method enables the detection of telomerase extracted from 18 ± 3 293T cells/μL.

Molecularly Imprinted Au Nanoparticles Composites on Au Surfaces for the Surface Plasmon Resonance Detection of Pentaerythritol Tetranitrate, Nitroglycerin, and Ethylene Glycol Dinitrate

Molecularly imprinted Au nanoparticles (NPs) composites are generated on Au-coated glass surfaces. The imprinting process involves the electropolymerization of thioaniline-functionalized Au NPs (3.5 nm) on a thioaniline monolayer-modified Au surface in the presence of a carboxylic acid, acting as a template analogue for the respective explosive. The exclusion of the imprinting template from the Au NPs matrix yields the respective imprinted composites. The binding of the analyte explosives to the Au NPs matrixes is probed by surface plasmon resonance spectroscopy, SPR, where the electronic coupling between the localized plasmon of the Au NPs and the surface plasmon wave leads to the amplification of the SPR responses originating from the dielectric changes of the matrixes upon binding of the different explosive materials. The resulting imprinted matrixes reveal high affinities and selectivity toward the imprinted explosives. Using citric acid as an imprinting template, Au NPs matrixes for the specific analysis of pentaerythritol tetranitrate (PETN) or of nitroglycerin (NG) were prepared, leading to detection limits of 200 fM and 20 pM, respectively. Similarly, using maleic acid or fumaric acid as imprinting templates, high-affinity sensing composites for ethylene glycol dinitrate (EGDN) were synthesized, leading to a detection limit of 400 fM for both matrixes.

Stereoselective and Chiroselective Surface Plasmon Resonance (SPR) Analysis of Amino Acids by Molecularly Imprinted Au‐Nanoparticle Composites

Au nanoparticles (NPs) functionalized with thioaniline and cysteine are used to assemble bis-aniline-bridged Au-NP composites on Au surfaces using an electropolymerization process. During the polymerization of the functionalized Au NPs in the presence of different amino acids, for example, L-glutamic acid, L-aspartic acid, L-histidine, and L-phenylalanine, zwitterionic interactions between the amino acids and the cysteine units linked to the particles lead to the formation of molecularly imprinted sites in the electropolymerized Au-NP composites. Following the elimination of the template amino acid molecules, the electropolymerized matrices reveal selective recognition and binding capabilities toward the imprinted amino acid. Furthermore, by imprinting of L-glutamic or D-glutamic acids, chiroselective imprinted sites are generated in the Au-NP composites. The binding of amino acids to the imprinted recognition sites was followed by surface plasmon resonance spectroscopy. The refractive index changes occurring upon the binding of the amino acids to the imprinted sites are amplified by the coupling between the localized plasmon associated with the Au NPs and the surface plasmon wave.

Optically activated uptake and release of Cu2+ or Ag+ ions by or from a photoisomerizable monolayer-modified electrode.

Di-(N-butanoic acid-1,8-naphthalimide)-piperazine dithienylethene was covalently linked to a cysteamine monolayer associated with a Au surface to yield a photoisomerizable monolayer composed of the open or closed dithienylcyclopentene isomers (3a or 3b), respectively. Electrochemical and XPS analyses reveal that the association of metal ions to the monolayer is controlled by its photoisomerization state. We find that Cu(2+) ions reveal a high affinity for the open (3a) monolayer state, K(a) = 4.6 × 10(5) M(-1), whereas the closed monolayer state (3b) exhibits a substantially lower binding affinity for Cu(2+), K(a) = 4.1 × 10(4) M(-1). Similarly, Ag(+) ions bind strongly to the 3a monolayer state but lack binding affinity for the 3b state. The reversible photoinduced binding and dissociation of the metal ions (Cu(2+) or Ag(+)) with respect to the photoisomerizable monolayer are demonstrated, and the systems may be used for the photochemically controlled uptake and release of polluting ions. Furthermore, we demonstrate that the photoinduced reversible binding and dissociation of the metal ions to and from the photoisomerizable electrode control the wettability properties of the surface.

Ultrasensitive and selective detection of alkaline-earth metal ions using ion-imprinted Au NPs composites and surface plasmon resonance spectroscopy

Au nanoparticles (NPs) functionalized with electropolymerizable thioaniline units and with dithiothreitol ligands were synthesized. The NPs were electropolymerized onto Au-coated glass surfaces in the presence of the alkaline-earth metal ions Mg2+, Ca2+, Sr2+ or Ba2+, to yield the respective ion-imprinted bis-aniline-bridged Au NPs composites on the Au surfaces. After elimination of the ions from the crosslinked matrices, specific imprinted ion recognition sites were generated in the composites. Selective association of the respective ions to the imprinted sites consisting of the dithiothreitol ligands is demonstrated. The high affinities of the metal ions to the respective imprinted sites lead to impressive sensitivities (fM range). The association of the ions to the imprinted sites is monitored by surface plasmon resonance spectroscopy, and the coupling between the localized plasmon of the NPs and the surface plasmon wave is used as an amplification mechanism.

Electrochemically triggered Au nanoparticles "sponges" for the controlled uptake and release of a photoisomerizable dithienylethene guest substrate.

1,2-Di(2-methyl-5-(N-methylpyridinium)-thien-3-yl)-cyclopentene undergoes a reversible photoisomerization between open and closed states. The closed isomer state exhibits electron acceptor properties, whereas its irradiation using visible light (λ > 530 nm) yields the open state that lacks electron acceptor features. The electropolymerization of thioaniline-functionalized Au nanoparticles (NPs) in the presence of the closed photoisomer state yields a molecularly imprinted Au NPs matrix, cross-linked by redox-active bis-aniline π-donor bridges. The closed isomer is stabilized in the imprinted sites of the bis-aniline-bridged Au NPs composite by donor-acceptor interactions. The electrochemical oxidation of the bis-aniline bridging units to the quinoid acceptor state leads to imprinted sites that lack affinity interactions for the binding of the closed state to the matrix, leading to the release of the closed photoisomer to the electrolyte solution. By the cyclic reduction and oxidation of the bridging units to the bis-aniline and quinoid states, the reversible electrochemically controlled uptake and release of the closed photoisomer is demonstrated. The quantitative uptake and release of the closed isomer to and from the imprinted Au NPs composites is followed by application of CdSe/ZnS quantum dots as auxiliary probes. Similarly, by the reversible photochemical isomerization of the closed substrate to the open substrate (λ > 530 nm) and the reversible photoizomerization of the open substrate to the closed state (λ = 302 nm), the cyclic photonic uptake and release of the closed substrate to and from the imprinted Au NPs matrix are demonstrated. Finally, we demonstrate that the electrochemically stimulated uptake and release of the closed substrate to and from the imprinted Au NPs composite controls the wettability of the resulting surface.