Photochemical switching of the phase-transition temperatures of p-NIPAM-Pt nanoparticles thermosensitive polymer composites associated with electrodes: functional electrodes for switchable electrocatalysis.

The thermosensitive poly(N-isopropylacrylamide) (p-NIPAM) is electropolymerized onto Au surfaces. The incorporation of the photoisomerizable N-carboxyethyl nitrospiropyran compound into p-NIPAM allows the reversible photochemical control of the gel-to-solid phase-transition temperatures of the polymer. Whereas the gel-to-solid phase-transition temperature of the nitrospiropyran-modified p-NIPAM is 33±2 °C, the phase-transition temperature of the nitromerocyanine-functionalized p-NIPAM matrix corresponds to 38±1 °C. Upon the incorporation of Pt nanoparticles (NPs) into the photochemically controlled p-NIPAM, a hybrid photoswitchable electrocatalytic matrix is formed. At a fixed temperature corresponding to 38 °C, the effective electrocatalytic reduction of H(2)O(2), or the oxidation of ascorbic acid, proceeded in the presence of the nitromerocyanine-functionalized p-NIPAM, yet these electrocatalytic transformations were inhibited in the presence of the nitrospiropyran-modified p-NIPAM.

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.

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.

Electrified Au nanoparticle sponges with controlled hydrophilic/hydrophobic properties.

Molecularly imprinted Au nanoparticle (NP) composites for the selective binding of the electron acceptors N,N'-dimethyl-4,4'-bipyridinium, MV(2+) (1), or bis-N-methylpyridinium-4,4'-ethylene, BPE(2+) (2), are prepared by the electropolymerization of thioaniline-functionalized Au NPs in the presence of the electron acceptor molecules and the subsequent rinsing off of the imprint substrates. The electrochemical oxidation of the π-donor bisaniline units bridging the Au NPs yields the quinoid electron acceptor bridges, which are re-reduced to the bisaniline state. By the cyclic oxidation and reduction of the bridging units, they are reversibly switched between the π-acceptor and the π-donor states, thus allowing the electrochemically triggered uptake and release of the electron acceptors (1 or 2) to and from the imprinted sites. While the electron acceptors 1 or 2 bind to the imprinted sites via donor-acceptor interactions, these substrates are released from the Au NP matrices upon the oxidation of the bridges to the quinoid state. The electrochemically switched wettability of the imprinted composites is demonstrated upon the reversible uptake, or release, of the substrates to and from the matrices. While the association of 1 or 2 to the respectively imprinted composites generates hydrophilic surfaces (θ = 30 and 41°, respectively), the release of the substrates from the matrices yields surfaces of enhanced hydrophobicity (θ = 60 and 55°, respectively). The electrochemically switched wettability is selective to the imprinted substrates and is amplified in the presence of the imprinted matrices as compared to the non-imprinted composites. The association of MV(2+) or BPE(2+) to the imprinted sites is further electrochemically characterized.

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.

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.

Selective and enantioselective analysis of mono- and disaccharides using surface plasmon resonance spectroscopy and imprinted boronic acid-functionalized Au nanoparticle composites.

A method was developed for the synthesis of molecularly imprinted Au nanoparticle (NP) composites on electrodes by electrochemical means. The resulting composites include specific recognition sites for mono- or disaccharides. The method is based on the formation of a boronate complex between the respective saccharide and the boronic acid ligands associated with the Au NPs. The electropolymerization of the Au NPs leads, after cleavage of the respective boronate esters, and removal of the saccharide, to specific recognition sites for the association of the imprinted monosaccharides or disaccharides. The binding of the saccharides to the imprinted sites is followed by surface plasmon spectroscopy (SPR). The changes in the refractive index of the Au NP composites upon the binding of the saccharides to the imprinted sites are amplified by the coupling between the localized plasmon associated with the NPs and the surface plasmon wave propagating on the Au surface. This leads to the highly sensitive stereoselective and chiroselective detection of monosaccharides and disaccharides.

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.

Electrified selective "sponges" made of Au nanoparticles.

Imprinted Au nanoparticle (NP) composites are assembled on Au surfaces by the electropolymerization of thioaniline-functionalized Au NPs in the presence of the imprint molecules, picric acid (1), N,N'-dimethyl-4,4'-bipyridinium (2), and N,N'-dimethylbipyridinium-4,4'-ethylene dichloride (3). The existence of pi-donor-acceptor complexes between the substrates (1-3) and the pi-donor thioaniline units associated with the Au NPs or the pi-donor bis-aniline bridges cross-linking the Au NPs on the electrode surfaces led to the formation of the imprinted sites. Upon elimination of the electron acceptors (1-3) from the Au NP matrices, molecular contours for the selective binding of the respective substrates are generated. The bis-aniline bridges linking the Au NPs in the composite exhibit quasireversible redox properties. At E < 0.12 V vs Ag quasireference electrode (QRE), the bridges exist in the reduced bis-aniline, pi-donor state, whereas at E > 0.12 V vs Ag QRE, the bridging units exist in the quinoid, pi-acceptor state. As a result, the potential-induced uptake and release of any of the pi-acceptor substrates 1-3 is accomplished. While at E < 0.12 V, the pi-acceptor substrates bind to the Au NP matrices through pi-donor-acceptor interactions, at E > 0.12 V, the bound substrates are released from the matrices, due to transformation of the bridging units to the quinoid pi-acceptor state, which lacks binding affinity for the substrates. The binding and release of the substrates 1-3 to and from the Au NP composites are followed by surface plasmon resonance (SPR) spectroscopy, and the quantitative assay of the uptake and release is monitored by the extent of fluorescence quenching of the solution-soluble fluorescent labels, meso-tetramethyl pyridinium porphyrin (TMPyP(4+)) or Zn(II)-meso-tetraphenylsulfonatoporphyrin (Zn-TPPS(4-)). The electrostimulated functions of the Au NP "sponges" are controlled by several means: (i) Imprinting of the molecular contours for 1-3 in the Au NP composites generates high-affinity binding sites for the imprinted substrates. This leads to higher contents of the bound substrates at the Au NP sponges, as compared to the nonimprinted Au NP composites, and to an impressive selectivity in the association of the imprinted substrates. (ii) The binding capacity of the Au NP composites is substantially improved by the electrosynthesis of the matrices on a rough Pt black support bound to the base Au electrode.

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.

Coupled electrochemical/photochemical patterning and erasure of Ag(0) nanoclusters on Au surfaces.

A nitrospiropyran (1a) monolayer-modified Au electrode provides an active surface for the photolithographic patterning of an electrode with Ag(0) nanoclusters. Illumination of the monolayer-modified Au surface through a mask yields nitromerocyanine (1b)-patterned domains on the Au electrode that bind Ag(+) ions. The electrochemical reduction of the Ag(+) ions yields stable Ag(0) nanoclusters on the patterned domains, and this enables the SEM imaging of the resulting microstructures. The photoisomerizable and electroactive nitromerocyanine/Ag(0) nanoclusters exhibit several important features: (i) By the electrochemical oxidation of Ag(0) nanoclusters to Ag(+) ions, the patterned metallic nanoclusters are erased but stored in the form of Ag(+) ions, confined to the patterned monolayer. The stored information can be retrieved by the electrochemical reduction of the Ag(+) ions. (ii) The photoisomerization of the nitromerocyanine (1b)-modified domains to the nitrospiropyran (1a) state (lambda > 475 nm) coupled to the electrochemical oxidation of the Ag(0) nanoclusters to Ag(+) enables the removal of Ag(+) from the surface and its restoration for new patterning. (iii) The orthogonal patterning of the Au surface with Ag(0) nanocluster structures is accomplished by the stepwise coupled photochemical/electrochemical patterning of the surface by two (or more) irradiation steps through the mask.

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.

Imprinting of molecular recognition sites through electropolymerization of functionalized Au nanoparticles: development of an electrochemical TNT sensor based on pi-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.

Photochemically controlled electrochemical deposition and dissolution of Ag0 nanoclusters on au electrode surfaces.

A photoisomerizable thiolated nitrospiropyran SP, (1a), monolayer is assembled on a Au electrode by the primary deposition of thiolated nitromerocyanine isomer 1b as a monolayer on the electrode, followed by the irradiation of the surface with visible light, lambda > 475 nm. The surface coverage of nitrospiropyran units (1a) on the electrode is 2 x 10-10 mole cm-2. Irradiation of the electrode with UV light, 320 nm < lambda < 360 nm, results in the nitromerocyanine, MR, monolayer on the electrode that binds Ag+ ions to the phenolate units. The Ag+ ions associated with the MR monolayer undergo cyclic reduction to surface-confined Ag0 nanoclusters, and reoxidation and dissolution of the Ag0 nanoclusters to Ag+ ions associated with the monolayer are demonstrated. The electron-transfer rate constants for the reduction of Ag+ to Ag0 and for the dissolution of Ag0 were determined by chronoamperometry and correspond to ketred = 12.7 s-1 and ketox = 10.5 s-1, respectively. The nanoclustering rate was characterized by surface plasmon resonance measurements, and it proceeds on a time scale of 10 min. The size of the Ag0 nanoclusters is in the range of 2 to 20 nm. The electrochemically induced reduction of the MR-Ag+ monolayer to the MR-Ag0 surface and the reoxidation of the MR-Ag0 surface control the hydrophilic-hydrophobic properties of the surface. The advancing contact angle of the MR-Ag0-functionalized surface is 59 degrees , and the contact angle of the MR-Ag+-monolayer-functionalized surface is 74 degrees . Photoisomerization of the Ag0-MR surface to the Ag0-SP state, followed by the oxidation of the Ag0 nanoclusters, results in the dissolution of the Ag+ ions into the electrolyte solution.

Cyclic control of the surface properties of a monolayer-functionalized electrode by the electrochemical generation of Hg nanoclusters.

Hg(2+) ions are bound to a 1,4-benzenedimethanethiol (BDMT) monolayer assembled on a Au electrode. Electrochemical reduction of the Hg(2+)-BDMT monolayer to Hg(+)-BDMT (at E degrees =0.48 V) and subsequently to Hg(0)-BDMT (at E degrees =0.2 V) proceeds with electron-transfer rate constants of 8 and 11 s(-1), respectively. The Hg(0) atoms cluster into aggregates that exhibit dimensions of 30 nm to 2 microm, within a time interval of minutes. Electrochemical oxidation of the nanoclusters to Hg(+) and further oxidation to Hg(2+) ions proceeds with electron-transfer rate constants corresponding to 9 and 43 s(-1), respectively, and the redistribution of Hg(2+) on the thiolated monolayer occurs within approximately 15 s. The reduction of the Hg(2+) ions to the Hg(0) nanoclusters and their reverse electrochemical oxidation proceed without the dissolution of mercury species to the electrolyte, implying high affinities of Hg(2+), Hg(+), and Hg(0) to the thiolated monolayer. The electrochemical transformation of the Hg(2+)-thiolated monolayer to the Hg(0)-nanocluster-functionalized monolayer is characterized by electrochemical means, surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact-angle measurements. The Hg(0)-nanocluster-modified surface reveals enhanced hydrophobicity (contact angle 76 degrees ) as compared to the Hg(2+)-thiolated monolayer (contact angle 57 degrees ). The hydrophobic properties of the Hg(0)-nanocluster-modified electrode are further supported by force measurements employing a hydrophobically modified AFM tip.

Switchable surface properties through the electrochemical or biocatalytic generation of Ag0 nanoclusters on monolayer-functionalized electrodes.

The electroswitchable and the biocatalytic/electrochemical switchable interfacial properties of a Ag(+)-biphenyldithiol (BPDT) monolayer associated with a Au surface are described. Upon the application of a potential corresponding to -0.2 V the Ag(+)-BPDT is reduced to the Ag(0)-BPDT interface, and silver nanoclusters are generated on the interface. The application of a potential that corresponds to 0.2 V reoxidizes the monolayer to the Ag(+)-BPDT monolayer. The reversible electrochemical transformation of the Ag(+)-BPDT monolayer and of the Ag(0)-BPDT surface was followed by electrochemical means and surface plasmon resonance spectroscopy (SPR). The SPR experiments enabled us to follow the kinetics of nanoclustering of Ag(0) on the surface. The hydrophobic/hydrophilic properties of the surface are controlled by the electrochemically induced transformation of the interface between the Ag(+)-BPDT and Ag(0)-BPDT states. The Ag(0)-BPDT monolayer reveals enhanced hydrophilicity. The hydrophobic/hydrophilic properties of the interface were probed by contact angle measurements and force interactions with a hydrophobically-functionalized AFM tip. The Ag(0)-BPDT interface was also biocatalytically generated using alkaline phosphatase, AlkPh, and p-aminophenyl phosphate as substrate. The biocatalytically generated p-aminophenol reduces Ag(+) ions associated with the surface to Ag(0) nanoclusters. This enables the cyclic biocatalytic/electrochemical control of the surface properties of the modified electrode.