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.

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.

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-1,3,5-triazine (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.[1,2] Different optical, electrochemical, or microgravimetric sensors or biosensors for TNT were reported. Fluorescent organic polymers, which are quenched by nitroaromatic explosives, [3,4] luminescent polysilole nanoparticles (NPs), [5,6] 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, [7,8] and recently, a composite of Au NPs linked to electrodes enabled a sensitive electrochemical detection of TNT.[9] Also, different sensing matrices, such as cyclodextrin polymers, [10] carbowax, [11] or silicon polymers, [12] were used for TNT analysis by surface acoustic wave devices, and the aggregation of functionalized Au NPs in the presence of TNT was used to develop an optical sensor for the explosive.[13] Similarly, antibody-based optical [14][15][16] or microgravimetric quartz-crystal-microbalance [17] biosensors for TNT were developed. Different optical [18,19] or voltammetric [20] sensors for RDX were also reported. These included the fluorescence detection of RDX with an acridinium dye, [18] or the application of NADH-functionalized quantum dots.[21] Also, a competitive fluorescence immunoassay for the detection of RDX was reported.[22] 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. [23,24] Numerous SPR sensors and biosensors were developed, [25][26][27] and metal NPs were implemented to enhance the SPR response and to amplify SPR-based sensors. [28,29] 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, [30] formation of immunocomplexes, [31] and the probing of biocatalytic transformations, [32] 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.[33] The formation of p-donor-acceptor complexes ...

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.

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.