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Herein, a smartphone-based portable reader with integrated optics for standard microtiter plates (96 wells) has been designed and demonstrated for high-throughput quantitation of validated biomarkers in serum. The customized optical attachment was simply constructed with a convex lens and a light source, by which the transmitted light through a 96-well microtiter plate was converged for imaging with a smartphone, so that accurate and wide-range reading of the plate can be achieved. More importantly, relying on the digitized colorimetric analysis of the obtained images, concentrations of various biomarkers can be determined directly using the customized mobile app. A set of validated biomarkers for inflammation and infection, C-reactive protein (CRP), serum amyloid A (SAA), and procalcitonin (PCT) have been quantitated with this new system; both the response ranges and limits of detection meet the requirement of clinical tests. The consistency with the results obtained using a commercial microplate reader proves its reliability and precision, augments its potential as a point-of-care diagnostic device for on-site testing or resource-limited settings.

Fingerprints consist of unique patterns of skin ridges and valleys, and are commonly described by three levels of feature details. Most fingerprint identification systems rely on matching only firs...

The macrocyclic cucurbit[7]uril (CB[7]) host has exhibited great application potential as a pharmaceutical excipient due to its versatile abilities to modulate the chemical/physical properties of drug molecules (guests) and to control their in vivo delivery and release (upon complexation). The formation of stable CB[7]@drug complexes is the prerequisite for these promising applications; we report herein a general assay strategy to quantitate the complexation based on competitive binding with surface-immobilized redox guests in conjunction with conventional electrochemical techniques (e.g., cyclic voltammetry). Particularly, by incubating a mixture of CB[7] and a drug molecule with ferrocene (Fc)-terminated self-assembled monolayers (SAMs) on gold, the competitive host@guest binding between the CB[7]@drug complex formed in solution and the CB[7]@Fc complex formed on surface can be quantified with direct cyclic voltammetry measurements. On the basis of the known concentrations of CB[7]/drug and electrochemically determined surface densities of free/complexed Fc groups, the formation constant of CB[7]@drug complex can be determined. With several drug molecules as examples, we have demonstrated the capability of this method for quantitative studies of the formation of supramolecular excipient@drug complexes that are of interest in pharmaceutical and biomedical sciences. More importantly, this work promises a general assay strategy that allows electrochemical quantitation of a wide range of electro-inactive analytes based on the competitive supramolecular host@guest binding at redox-tagged molecular interfaces.

We demonstrate herein that hemin–graphene hybrid nanosheets (H–GNs) can differentiate hemin-binding DNA aptamers (i.e., guanine-rich DNA sequences that form G-quadruplex structures upon binding hemin) and nonspecific single-stranded oligonucleotides colorimetrically. By exploiting the tendency of the H–GNS to aggregate under high ionic strength and their inherent catalytic activities, excess H–GNS were removed and the amount of H–GNs remaining in the supernatant was quantified. Subsequently, the binding affinity of hemin-binding DNA aptamers to the H–GNS was determined upon titration with the DNA. With the conformity of the experimental data to the Langmuir adsorption isotherm, hemin is hypothesized to be sandwiched between the G-quadruplex DNA and graphene forming a ternary complex as a result of the specific binding of the aptamer with the H–GNS. The binding affinity of the aptamer is not as strong as that of hemin/G-quadruplex in solution, which is attributed to electronic effects imparted by the graphene surface. The catalytic activity of the resulting complex, however, was confirmed to be higher than that of H–GNs alone.

The design and testing of integrated colorimetric microarray immunochips (immuno-microarrays) are reported for the quantitation and direct visual determination of multiple illicit drugs (e.g., morphine, cocaine and amphetamine) in body fluids. Such an immuno-microarray platform utilizes a competitive immunoassay format, which is based on silver staining for quantitative detection and multicolor staining for direct visualization (i.e., qualitative identification) of analytes present in the sample. Under optimized conditions, the dynamic response ranges of 3.7–1000, 1.1–300 and 1.5–300 ng mL−1 were achieved for amphetamine, cocaine, and morphine, respectively, which are wider towards low concentrations than those of standard enzyme-linked immunosorbent assay (ELISA) tests. The limits of detection (LODs) for morphine, cocaine, and amphetamine were determined to be 1.5 ± 0.1, 1.1 ± 0.1 and 3.7 ± 0.2 ng mL−1, respectively in oral fluids, which meet government regulations for law enforcement. The obvious advantages of multiplexing, simultaneous visual recognition, and accurate quantitation make the on-site detection feasible, confirming that such a colorimetric immuno-microarray holds promise for practical applications.

The influence of surface crystallography and applied potential on the thiol-exchange procedure to create mixed alkylthiol DNA SAMs is detailed. A single crystal gold bead and fluorophore labeled thiol modified DNA were used to characterize the resulting surface modifications. The thiol-exchange occurs with different efficiencies on the low index planes (111,100,110) as compared to 311 and 210. Positive applied potentials (>0/SCE) result in 10 × higher coverage than when compared to deposition at the open circuit potential (OCP) over the same 60 min time period. Negative potentials ( 0/SCE) resulted in less uniform coverage with the 111 facet being significantly modified. The electrolyte used during the deposition was a 10 mM TRIS Buffer with 100 mM NaCl 500 mM MgCl 2 . The influence of [ Cl − ] was studied showing it had a significant impact on the thiol-exchange at the positive potentials, where higher [ Cl − ] resulted in higher DNA coverages and a more uniform coverage across the multi-crystalline surface. The local environment of the thiol-exchanged DNA SAMs were compared for different regions on the surface using potential driven DNA reorientation modulating the fluorescence intensity. These results showed a common behaviour from all surfaces suggesting that the DNA SAMs prepared by thiol-exchange were consistently prepared with a variable surface concentration controlled by potential and time.

The electrocatalytic oxygen evolution reaction (OER) requires stable, highly active, and robust earth-abundant electrocatalysts. In the present study, we combine theoretical simulations with experimental approaches to design and synthesize NiFeW layered double hydroxide (LDH) for efficient OER in alkaline electrolytes. Density functional theory with the Hubbard U (DFT + U) calculations suggests that W, a high-valence metal, can improve the catalytic activity of Fe sites and optimize adsorption energies for OER intermediates at the surface of NiFeW LDH. Electrochemical measurements of the as-synthesized NiFeW LDH with a constant mass loading reveals that the LDH with the NiFeW molar ratio of 4:1:1 presented the highest intrinsic OER activity among all bi- and trimetallic LDH catalysts in this study. Furthermore, the nanostructures with the optimal active metal molar ratio are in situ grown on hydrophilic-treated carbon paper to fabricate an integrated 3D electrode with an overpotential of 248 mV at the catalytic current density of 20 mA cm−2 and a low Tafel slope of 68 mV dec−1 in 1.0 M KOH solution. This indicates that NiFeW LDH is among the most active NiFe-based OER electrocatalysts reported to date. X-ray photoelectron spectroscopy and elemental analysis confirm that the optimized Ni Fe W LDH presents a stable chemical composition and Ni, Fe, and W are still present in the catalyst after alkaline OER measurements. These findings offer new insights and avenues for the future design and study of stable and active multi-metal-based OER electrocatalysts.

Graphene as a highly conductive, adsorptive, and structurally flexible material has been widely employed to develop advanced electronic devices and sensors. In this study, by simply drop-casting the desired amount of industrial grade, graphene nano-platelets onto conventional double-sided conductive tape, we developed a convenient, versatile, and scalable graphene substrate (namely, flexible graphene substrate in a snap) with much-improved in-plane conductivity for electrochemical analysis and template-assisted deposition of functional nanostructures. Particularly cyclic voltammetric measurements with reversible redox couples showed ideal responses on such “flexible graphene electrodes” with superior reproducibility and stability. The quantitation of medically relevant, trial analysts (i.e., ascorbic acid and dopamine) showed excellent linearity in established calibration curves (R2 > 0.995) and satisfactory detection sensitivity. More remarkably, by affixing nano-porous templates (polycarbonate or AAO membranes) onto these flexible graphene substrates, we can fabricate various metallic nanostructures via a conventional bench-top electrode position process, which exhibit excellent electro-analytical (Au nano-clusters as ideal ultra-microelectrode arrays) and catalytic activities (Pt nanowires towards modulated H2O2 reduction). Beyond the application of quantitative electrochemical analysis and bench-top creation of functional nanostructures, we envision that such “flexible graphene substrate in a snap” as a versatile, scalable platform material can be adapted to many other designs of graphene-based sensors and analytical devices.

The template-controlled electrodeposition has been widely used for fabricating arrayed micro-/nanostructures (both metallic and composite), which typically requires the vacuum-deposition of a conductive metal film on the template and faces the challenge of detaching them from the substrate electrode for subsequent characterization/application. Herein we report that such a template-assisted fabrication strategy can be readily accomplished by using double-sided carbon tapes in conjunction with nanoporous templates of choice, which allows for not only simple, rapid assembly with any substrate electrode for the deposition of arrayed micro-/nanostructures with standard electrochemical equipment, but also their effort-less detachment for subsequent characterization and application. As of the proof-of-concept, we have demonstrated that “hemisphere” gold ultramicroelectrode arrays with characteristic redox responses can be reproducibly prepared in a few minutes with appropriate deposition time and potential. The application of this convenient method as a routine, benchtop protocol for preparing different types of arrayed micro-/nanostructures (e.g., Au, Cu, and Pt) using various nanoporous templates (both polycarbonate membranes and anodic aluminum oxide substrates) via conventional electrochemical deposition has been also described.

New methods to detect copper sensitively and specifically are of practical importance because of its presence in both ecological and biological systems. Herein, we report a newly selected DNAzyme, CLICK-17, which catalyzes the azide-alkyne cycloaddition reaction strictly dependent on the presence of copper, as a superior transducer for electrochemical copper quantitation. CLICK-17 offers the following notable advantages: it does not require the presence of an unstable reducing agent (detects both Cu(II) and Cu(I) in samples); it offers rapid detection owing to the fast in cis coupling kinetics; and, it generates no background because its catalytic activity strictly relies on the presence of copper (signal-on). For performing electrochemical sensing, the DNAzyme efficiently couples to a binary 11-azido-1-undecanethiolate/octanethiolate monolayer on gold (N3C11S-/C8S-Au), which is then accurately quantitated by the cyclic voltammetric response of electrostatically bound [Ru(NH3)6]3+. As the negative charges on the phosphate backbone of surface-bound 79nt-DNA strands (CLICK-17) are inclusively compensated by the multiply charged redox cations (e.g., [Ru(NH3)6]3+) at low ionic strength, the electrochemical signal is naturally amplified (~ 30 folds). Thus constructed biosensor detects Cu(II) with a LOD of 3.5 nM and Cu(I) with a LOD of 0.8 nM, which is also free of background signals and highly specific against other metal ions. More importantly, we have shown the successful application of this DNAzyme-based electrochemical sensor to determine copper concentrations in natural ores from the mining industry, with performance comparable to the gold standard flame atomic absorption spectrophotometry.

Graphene oxide (GO)-based aptasensors are currently one of the most popular sensing platforms for the simple and rapid detection of various targets. Unfortunately, the GO-based aptasensors with long aptamer strands typically show unsatisfactory performance resulting from insignificant structural transformations upon target binding. We report herein the utilization of an aptamer-truncating strategy to combat such a challenge. Taking a pre-selected anti-aflatoxin B1 (AFB1) aptamer (P-AFB1-50) as a trial system, we sequentially remove the extraneous nucleotides within the aptamer by means of circular dichroism (CD) spectroscopy and binding affinity analysis. Particularly, the ratio of the quenching constants between the GO sheets and the truncated aptamers (labelled with fluorophores) in the absence and presence of the target was determined for each of the truncated aptamers to evaluate the optimal sequence. As a result, the truncated aptamer comprising 40 nucleotides was confirmed to show the highest FL output and the best detection limit upon conjugation with GO sheets. More importantly, we demonstrated that this truncating strategy is versatile, i.e., it can be easily extended to other aptamer systems (anti-ochratoxin A (OTA) aptamer, P-OTA-61, as an example) for extraneous nucleotide identification. Impressively, the two optimal truncated aptamers can work together on GO sheets to achieve a simultaneous detection of two different mycotoxins (i.e., AFB1 and OTA) in one single test. Essentially, this research opens a new avenue for the design and testing of aptamer-/GO-based-sensing platforms for rapid, low-cost and multiplex quantification of analytical targets of interest.

The preparation of DNA self-assembled monolayers (SAMs) on single-crystal gold bead electrodes using an applied potential is evaluated with in situ electrochemical fluorescence microscopy. Applying a constant deposition potential or a square-wave potential perturbation during the formation of DNA SAMs is compared for two different modification methods: DNA SAM formation on a clean gold surface followed by alkythiol backfilling (as is typically done in the literature) or via thiol-exchange on an alkylthiol-modified gold surface. DNA SAMs prepared from a chloride-containing deposition buffer were not significantly different when using either square-wave potential perturbation or at a constant applied potential even when considering different surface crystallographies. Greater variations were observed when applying more positive potentials for both DNA thiol-exchange and DNA adsorption on clean Au. Our results suggest that using either a constant potential or a square-wave potential perturbation for 5 min both create defects by weakening the gold-thiol interaction. When the deposition is performed with the adsorption of chloride ions from the electrolyte, the electrodeposition results in a similar increase in DNA coverage when compared to depositions performed at open circuit potentials.

Typically, DNA SAMs used for electrochemical based sensing are prepared without controlling the gold electrode potential[1]. Applying a potential during DNA deposition enables control over the coverage and DNA organization[2-4]. These characteristics are determined by the composition of the immobilization buffer, the immersion time, the DNA concentration, the extent of backfilling with a small alkylthiol and whether deposition occurs on a bare or alkythiol coverage gold surface[3-4]. Few studies have explored the influence of the underlying surface crystallography. Here, we show the possibilities of using electrochemical potential control to manipulate the characteristics of the DNA SAM. Moreover, the use of a single crystal gold bead electrode, coupled with the use of a fluorophore tagged DNA and fluorescence microscopy, enables examination of the influence of the underlying surface crystallography on the DNA SAM prepared. Various constant potentials were used as well as using a square-wave potential perturbation during deposition. There are significant differences in the DNA SAM produced under these conditions. In some cases, the DNA coverage was low on the surfaces, higher on more open or atomically rough regions. In other cases, the coverage was higher on the surfaces as compared to the other regions. The use of constant positive or negative potentials (vs SCE) resulted in significant differences in the relative coverages among regions of different atomic arrangements (Fig 1). In addition, a significant difference was observed between immobilization buffers that contain Cl and those that are Cl free. Also, the use of constant or square-wave potential perturbations resulted in significantly different local organizations of DNA in the SAM. Examples of these types of DNA SAM surfaces will be presented with some conclusions on the implications for use on polycrystalline planar gold surfaces. References: [1] Xiao, Y.; Lai, R. Y.; Plaxco, K. W. Preparation of Electrode-Immobilized, Redox-Modified Oligonucleotides for Electrochemical DNA and Aptamer-Based Sensing. Nat. Protoc. 2007, 2 (11), 2875–2880. [2] Jambrec, D.; Kayran, Y. U.; Schuhmann, W. Controlling DNA/Surface Interactions for Potential Pulse‐Assisted Preparation of Multi‐Probe DNA Microarrays. Electroanal. 2019, 410, 3. [3] Leung, K. K.; Gaxiola, A. D.; Yu, H. Z.; Bizzotto, D. Tailoring the DNA SAM Surface Density on Different Surface Crystallographic Features Using Potential Assisted Thiol Exchange. Electrochim. Acta 2018, 261, 188–197. [4] Leung, K. K.; Yu, H. Z.; Bizzotto, D. Electrodepositing DNA Self-Assembled Monolayers on Au: Detailing the Influence of Electrical Potential Perturbation and Surface Crystallography. ACS Sens. 2019, 4 (2), 513–520. Figure 1: Figure 1

The properties of DNA self-assembled monolayers (SAMs) have strong influences on the interfacial DNA–analyte binding behavior, which further affect the performance of biosensors built upon. In this work, we prepared binary thiolate DNA/6-ferrocenyl-1-hexanethiol (FcC6SH) SAMs on gold (DNA/FcC6S-Au) for convenient electrochemical characterization and subsequent data analysis. Our cyclic voltammetric (CV) studies confirmed that the redox responses of surface-tethered Fc and electrostatically bound [Ru(NH3)6]3+ are capable of providing quantitative information regarding the DNA film properties, including the surface density, structural heterogeneity, and molecular orientation under different preparation and measurement conditions. With the binary thiolate DNA/FcC6S-Au SAM prepared in the conventional post-assembly exchange protocol as a trial system, we are demonstrating the capability of introducing redox-active thiols as passivating and labeling reagents for preparing many other DNA-based biosensing interf...

The complete formation of stem-loop (i.e., hairpin) configuration on chip surface is of particular importance for the application of hairpin DNA (hpDNA) in building biosensors for various analytes with optimized performance. We report herein a convenient electrochemical protocol for evaluating the yield of hairpin DNA conformations upon self-assembly on electrode surface. As of the different hydrolysis capability of Exonuclease I (Exo I) toward single-stranded DNA (ssDNA) and hpDNA, we can selectively remove ssDNA from electrode but retain hpDNA strands; based on the changes in the cyclic voltammetric (CV) responses using [Ru(NH3)6]3+ as redox indicators, we can then determine the fraction of hairpin configurations in mixed DNA self-assembled monolayers (SAMs). It was discovered that the molar fraction of hairpin configuration formed on the surface is considerably lower than that in the binary deposition solution (containing both ssDNA and hpDNA). The accuracy of the Exo I-assisted electrochemical quantit...

The redox properties of electroactive self-assembled monolayers (SAMs) on gold in the organic phase were investigated by using the thin-film voltammetry approach, i.e., imposing a thin layer of nitrobenzene between the electrode and the bulk aqueous electrolyte. It was discovered that the structures of binary Fc(CH2)11S-/CH3(CH2)10S-Au SAMs became more homogeneous than in aqueous medium, as indicated by the appearance of a single pair of redox peaks even at high ferrocenylalkanethiolate surface densities. Intermolecular interactions became much weaker and eventually negligible (at low Fc surface density) as confirmed by simulating the experimental cyclic voltammograms (CVs) based on theoretical i-E relationship upon considering a Frumkin isotherm. The formation constant of i Fc + ⋅ Cl O 4 − on pairs in the organic phase was found to be much larger than that in aqueous medium, which reaffirms the predominant influence of the microenvironment around the redox centers (organic medium vs. aqueous electrolyte).

The correlation between redox properties and structural nature in a complete set of mono-ferrocenylpyrimidine derivatives (2-ferrocenylpyrimidine, 2-FcPy; 4-ferrocenylpyrimidine, 4-FcPy; 5-ferrocenylpyrimidine, 5-FcPy) was evaluated by investigating the intramolecular electronic communications. Both conventional electrochemical measurements in organic solvents and thin-film voltammetric studies of these compounds were carried out. It was discovered that their formal potentials are significantly different from each other, and shift negatively in the order of 4-FcPy > 5-FcPy > 2-FcPy. This result suggests that the intramolecular electronic communication is dictated by the delocalization effect of the π-bonding systems in 2-FcPy, and that the electron-withdrawing effect of the nitrogen atoms in the pyrimidine ring plays the key role in 4-FcPy and 5-FcPy. The single crystal X-ray structure analyis and Density Functional Theory (DFT) calculation provided additional evidence (e.g., different torsion angles between the cyclopentadienyl and pyrimidine rings) to support the observed correlation between the redox properties and structural nature.

Two novel starburst donors based platinum acetylide sensitizers with π–π conjugated span group (2-vinylthiophene) were synthesized and applied in dye-sensitized solar cells (DSSCs). The photovoltaic performances of sensitizers were investigated by photophysical, photovoltaic, electrochemical, and a theoretical approach. Compared with similar triphenylamine based platinum acetylide sensitizers which do not contain a 2-vinylthiophene group, it was found that the introduction of 2-vinylthiophene as a span group increased the donating ability of donor and the coplanar property between the donor and π-bridge, which are in favour of the intramolecular charge transfer. Moreover, the new DSSCs based sensitizers displayed a power conversion efficiency of 2.80% and 2.95%, which is significantly improved over that determined for the non-vinylthiophene containing analogues.

Ferrocenylalkanethiols are excellent probes to study the structure and properties of mixed self-assembled monolayers (SAMs) on gold; in this paper, the molecular heterogeneity in binary redox-active SAMs on gold prepared via postassembly exchange and coadsorption processes is revealed electrochemically. The exchange process of single-component 11-ferrocenyl-1-undecanethiolate SAMs on gold (FcC11S–Au) with 1-undecanethiol (C11SH) is first investigated; it is shown that a single pair of redox peaks can be obtained upon prolonged immersion in C11SH/ethanol solution. For the coadsorption of FcC11SH and C11SH on gold, the splitting of the redox peak diminishes when the molar ratio FcC11SH decreases to

Beyond their essential applications in portable data storage for the past 30 years, optical discs and corresponding recording/reading technologies have been extensively explored with the ultimate goal of creating novel analytical tools for on-site chemical analysis and point-of-care (POC) medical diagnosis. In particular, the disc media (CD, DVD, and BD) are proven to be inexpensive and versatile substrate materials for the preparation of various biochips and microfluidic systems; conventional computer drives and disc players are widely adapted for biochip signal reading and microscopic imaging. Herein we provide an overview of such optical disc technology-enabled analytical devices, e.g., integrated systems developed from specifically fabricated analog disks, modified optical drives, or adapted software algorithms.

We report herein an aptamer-based electrochemical biosensor for the quantitative determination of mucin 1 (MUC1), a glycoprotein expressed on most epithelial cell surfaces, based on the analyte-binding induced, global-scale conformational change of electrode-bound anti-MUC1 DNA aptamers. The modified electrode was readily prepared by directly immobilizing thiolated, single-stranded anti-MUC1 DNA aptamers on gold; they were also tethered at the distal end with methylene blue (MB) for electrochemical measurements. In the absence of MUC1, these DNA single strands fold into a thermodynamically stable “hairpin” conformation, which facilities direct electron transfer between MB and the gold electrode. Upon MUC1 binding, the aptamer no longer retains its inherent hairpin conformation, which relocates the redox center (MB) further away from the electrode surface. The surface density of the DNA aptamer probes on gold measured by MB voltammetric response is consistent with that of electrostatically bound [Ru(NH3)6]3+, indicating that every MB molecule is electroactive at low scan rates; yet, the electroactivity of MB decreases remarkably upon binding MUC1. The detection limit (50 nM) and dynamic response range (up to 1.5 μM) are superior to the commercially available enzyme-linked immunosorbent assay (ELISA) kits and our previously developed quantum dot-based fluorescence protocol. The sensing principle pertaining to the specific folding conformation described in this paper has the potential to serve as a general approach of using hairpin DNA aptamers to construct biosensors for other molecular analytes.

An ultra performance liquid chromatography with electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) method was developed for the simultaneous determination of 29 veterinary drugs of quinolones, sulfonamides, macrolides and nitrofurans in compound feeds. The sample was extracted with methanol-acetonitrile ( 1 : 1, v/v), and then cleaned up using solid phase extraction with an Oasis HLB column, detected by UPLC-ESI-MS/ MS. The UPLC separation was performed on a C18 column (100 mm x 2.1 mm, 1.7 microm) using a gradient elution with the mobile phases of methanol and 0. 1% (v/v) formic acid aqueous solution. The ESI-MS/MS detection was achieved in positive mode under multiple reaction monitoring (MRM) mode. The linear ranges were from 0.01 to 5.0 mg/L with the correlation coefficients above 0. 99 for all the 29 veterinary drugs. The average recoveries of the 29 veterinary drugs spiked in three feed matrixes (the compound premix feed, the complete formula feed and the concentrated feed) at four spiked levels were 61.2% - 94.3% with the relative standard deviations (RSDs) of 2.2% - 15.0%. The limits of detection were 0. 01 mg/kg or 0.05 mg/kg. The method is rapid, accurate, reproducible, sensitive and easy to apply, and suitable for the simultaneous determination of diverse veterinary drugs in compound feeds.

The redox behavior of two novel multicenter redox molecules (triferrocenylmethane and triferrocenylmethanol) has been studied in a thin film of nitrobenzene (NB) imposed between a graphite electrode and an aqueous electrolyte. The well separated three sets of redox peaks indicate strong intramolecular electronic communications between the three ferrocene centers in each molecule. They were adapted as model compounds for the study of electron transfer kinetics across the liquid/liquid interface with varied overall driving force using only one-type redox couples in the organic and aqueous phase, respectively. It has been shown that in both cases the dependence of interfacial electron transfer rate on the increased overall driving force across the nitrobenzene/water interface is not monotonic.

Phosphonate self-assembled monolayers (SAMs) have been widely used for the surface modification of indium–tin oxide (ITO) electrodes; however, their formation kinetics and stability are not well understood. In this paper, we describe our electrochemical studies of the formation kinetics and stability of a series of phosphonate SAMs on ITO electrodes. In particular electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) measurements have been carried out on three carboxy-terminated phosphonate SAMs by using Fe(CN) 6 3−/4− as redox indicators. The dependence of the charge-transfer resistance (obtained from the EIS plots) on the incubation time allows an estimation of the apparent fractional surface coverage of phosphonate SAMs. The apparent formation rate constant ( k obs ) was determined by fitting the experimental data to a Langmuir adsorption model. For 3-phosphonopropanoic acid (PPA), the k obs value increases when the PPA concentration increases in the deposition solution, and is smaller than those of thiolate SAMs on Au. The stability of phosphonate SAMs was investigated in three different media (pure water, phosphate-buffered saline (PBS) solution, and ambient air condition). It has been shown that the phosphonate SAMs are rather stable in either PBS solution or ambient air condition.

This work emphasizes on the design criteria of aptamer-based biosensors for adenosine and the comparison of their performances by using both electrochemical and biochemical methods. The adenosine biosensors have been typically prepared by hybridizing a short bridging DNA (modified with a thiol for the attachment to gold electrode) to the aptamer strand that releases upon adenosine binding. We have carefully compared the two “isomeric” designs, one with the thiol-linker tethered to the 3′-hydroxyl and the other to the 5′-phosphate end of the bridging DNA. It has been demonstrated through electrochemical and gel electrophoresis experiments that the 3′-thiol bridged sensor shows much better performance than its 5′-counterpart on surface, whereas in solution phase the two sensor systems are equally sensitive. We believe that the “transient” secondary structure of the aptamer sequence restricts the freedom of the strand displacement upon analyte binding, leading to a sluggish release of the aptamer strand from the 5′-thiol-bridging DNA.

Three arylferrocene derivatives, ferrocenylbenzene (MFcB), 1,3-diferrocenylbenzene (DFcB), and 1,3,5-triferrocenylbenzene (TFcB), were prepared and their redox properties systematically explored by thin-layer cyclic voltammetry (CV) and differential-pulse voltammetry (DPV). In contrast to conventional CV measurements that produced only a single pair of redox waves for all three compounds, the thin-layer technique discriminated between the multistep electron-transfer processes of DFcB and TFcB. In particular, two and three pairs of symmetric peaks were observed, respectively, when CV curves were recorded at a graphite electrode coated with a DFcB-containing and a TFcB-containing thin film of nitrobenzene and immersed in aqueous sodium perchlorate solution. These results demonstrate that the ferrocenyl moieties attached to the meta-positions of a benzene ring communicate electronically with each other, as a result of their distinct face-to-face orientations.

The bare LiFePO 4 and LiFePO 4 /C composites with network structure were prepared by solid-state reaction. The crystalline structures, morphologies and specific surface areas of the materials were investigated by X-ray difrractometry(XRD), scanning electron microscopy(SEM) and multi-point brunauer emmett and teller(BET) method. The results show that the LiFePO 4 /C composite with the best network structure is obtained by adding 10% phenolic resin carbon. Its electronic conductivity increases to 2.86 × 10 −2 S/cm. It possesses the highest specific surface area of 115.65 m 2 /g, which exhibits the highest discharge specific capacity of 164.33 mA·h/g at C /10 rate and 149.12 rnA·h/g at 1 C rate. The discharge capacity is completely recovered when C /10 rate is applied again.

We have thoroughly compared cyclic voltammetry and chronocoulometry, the two commonly used electrochemical techniques in quantitative determination of DNA surface density on gold electrodes. The electrostatic binding of redox cations, such as [Ru(NH 3 ) 6 ] 3+ , to the anionic DNA phosphate backbone allows the determination of the number of phosphate residues and thereby the surface density of the oligonucleotide strands. The results obtained by the two methods agreed well with each other for thiol-terminated DNA immobilized on gold electrodes via Au–S linkages and for unmodified DNA physically adsorbed on the surface. An improved procedure has been developed for the preparation of DNA-modified gold chips with a relatively high surface coverage. The density of thiol-terminated single-stranded DNA probes was determined to be (2.2 ± 0.3) × 10 13 molecules/cm 2 upon reaching adsorption equilibrium, which was in good agreement with that obtained by phosphor imaging of 32 P-radiolabeled DNA.

In order to reveal the relationship between the dipole moment and electric properties of organically modified semiconductor, a series of ω-functionalized alkyl monolayers on oxide-free silicon ( Si–(CH 2 ) 10 COOH, Si–(CH 2 ) 11 CH 3 , Si–(CH 2 ) 10 COOC 2 H 5 , and Si–(CH 2 ) 11 OH) was prepared and characterized. Electrochemical impedance measurements showed excellent insulating effects of these monolayers. It has been demonstrated that the dielectric constants of these organic monolayers are affected by the dipole moments introduced by the different terminal-groups. Solid-state electrical measurements showed that mercury | ω-functionalized alkyl monolayer | silicon junctions exhibit molecular tunability and a clear correlation between ideality factor and the film thickness/dielectric constant ratio. The barrier height is approximately proportional to the dipole moment of the monolayer. These results augment the possibility of fine-tuning the electrical properties of silicon-based microelectronic devices using functionalized organic monolayers.

In this paper, we present a smartphone-readable barcode assay for the qualitative detection of methyl parathion residues, a toxic organophosphorus pesticide that is popularly used in agriculture worldwide. The detection principle is based on the irreversible inhibition of the enzymatic activity of acetylcholinesterase (AchE) by methyl parathion; AchE catalytically hydrolyzes acetylthiocholine iodine to thiocholine that in turn dissociates dithiobis-nitrobenzoate to produce a yellow product (deprotonated thio-nitrobenzoate). The yellow intensity of the product was confirmed to be inversely dependent on the concentration of the pesticide. We have designed a barcode-formatted assay chip by using a PDMS (polydimethylsiloxane) channel plate (as the reaction reservoir), situated under a printed partial barcode, to complete the whole barcode such that it can be directly read by a barcode scanning app installed on a smartphone. The app is able to qualitatively present the result of the pesticide test; the absence or a low concentration of methyl parathion results in the barcode reading as “−”, identifying the test as negative for pesticides. Upon obtaining a positive result (the app reads a “+” character), the captured image can be further analyzed to quantitate the methyl parathion concentration in the sample. Besides the portability and simplicity, this mobile-app based colorimetric barcode assay compares favorably with the standard spectrophotometric method.

A microarray-format colorimetric biochip was constructed on plastic using two specially-designed DNA hairpin strands as binary probes and a binding-induced conformational switching strategy as the signal generation protocol. Coupled with single- or dual-color staining, we were able to simultaneously detect and quantitate the trace amounts of Pb(2+) and Hg(2+) in various real-world samples.

The redox behavior of 1,3,5-triferrocenylbenzene (TFcB), a hydrophobic molecule with three structurally equivalent redox centers, was explored by thin-layer cyclic voltammetry (CV). Two pairs of redox waves were observed when the CV was recorded at a graphite electrode coated with a TFcB-containing thin film of nitrobenzene and immersed in aqueous sodium perchlorate solution, in contrast to the single pair of redox peaks obtained in conventional CV measurements. These results suggest that the redox reactions of TFcB occur in two distinct steps involving a total of three electrons, and that electronic intramolecular communication exists among the three ferrocenyl units. Keywords: Thin-layer electrochemistry, Cyclic voltammetry, 1,3,5-Triferrocenylbenzene, Electronic intramolecular communication

Here, we report a novel route to prepare bioreactive surfaces on gold by the self-assembly of generation-three hydroxyl-terminated dendron thiols (G3-OH) and subsequent bridging reactions using generation-two amine-terminated dendrimers (G2-NH(2)). It has been shown that G3-OH dendron thiols form a stable and uniform self-assembled monolayer on gold, which can be activated by the homobifunctional cross-linker N,N-disuccinimidyl carbonate (DSC). Subsequent derivatization of the activated monolayer via dendrimer bridging reactions with G2-NH(2) enhances the stability, reactivity, and versatility of the prepared surface. Each step of the surface formation reaction has been monitored, and the resulting surface has been characterized by wetting, electrochemistry, scanning tunneling microscopy (STM), and infrared (IR) spectroscopy measurements. The reactivity of this surface was demonstrated by a Schiff base coupling reaction with 4-cyanobenzaldehyde, by immobilizing biotin molecules onto the peripheral amine groups using one of the conjugation methods, and by further binding avidin onto the biotinylated surface. We believe that the prepared bioreactive surface with a high density of amine groups will be useful for the immobilization of biological macromolecules for various biosensor applications, such as the fabrication of DNA microarrays and protein chips.

The ion-exchange kinetics of metal cation binding to and dissociation from thiolate-DNA monolayers on gold can be monitored by a simple electrochemical protocol. The apparent first-order rate constants were obtained by analyzing the time-dependent voltammetric behavior of the redox cation [Ru(NH3)6]3+. It was found that the binding kinetics is dominated by the structural nature of the film; i.e., the apparent first-order rate constant (kapp) decreases significantly upon increasing the surface density of DNA strands. Dissociation rate constants were obtained by transferring the incubated electrode into redox-free buffer solution. The kinetic data augment our fundamental understanding of metal ion-DNA interactions and are critical to ensure the accuracy and reliability of experimental DNA detection protocols.

The structure, reactivity, and acid-base properties of mixed monolayers prepared by photochemical reaction of hydrogen-terminated silicon with mixtures of ethyl undecylenate and n-alkenes were studied by ATR-FTIR spectroscopy and contact-angle measurements. The surface composition of the mixed monolayers and its correlation with the hydrolysis reactivity of terminal ethoxycarbonyl (ester) groups were investigated by systematically varying the mole fraction of ethyl undecylenate and the chain length of the unsubstituted alkenes in the binary deposition solution. It has been shown that the mole fraction of ester groups on the surface deviates only slightly from the mole fraction of ethyl undecylenate in the solution. The efficiency of ester hydrolysis under acidic conditions is significantly influenced by the monolayer structure, i.e., the surface density of ester groups and length of the unsubstituted alkyl chains. In addition, we find that mixed omega-alkanoic acid/alkyl monolayers on silicon (prepared via hydrolysis) exhibit well-defined contact angle titration curves from which the surface acid dissociation constants were determined. The results were compared with the acid-base properties reported in the literature for carboxylic acid-terminated alkylsiloxane monolayers on hydroxylated silicon and for omega-mercaptoalkanoic acid/alkanethiolate monolayers on gold. The weak pKa dependence (deltapKa approximately 1) on the surface density of carboxylic acid groups and on the length of unsubstituted alkyl chains is attributed to variations of the microenvironment of the acid moieties. These experimental findings provide fundamental knowledge at the molecular level for the preparation of bioreactive surfaces of controlled reactivity on crystalline semiconductor substrates.

To examine DNA-modified surfaces, we have developed a simple, convenient, and reliable procedure based on the voltammetric response of multiply charged transition metal cations (such as [Ru(NH3)6]3+) bound electrostatically to the DNA probes. At micromolar concentrations of the redox molecules in the electrolyte, the reduction and oxidation waves resulting from the immobilized cations on DNA-modified electrodes are well defined, stable, and reproducible. The surface densities of both single- and double-stranded oligonucleotides were accurately determined by integration of the peak for reduction of [Ru(NH3)6]3+ to [Ru(NH3)6]2+. In addition, the binding constant and electron-transfer rate constant of [Ru(NH3)6]3+ on DNA-modified electrodes were evaluated with the help of classical models. The present research provides not only an applicable and simple protocol for the quantitation of DNA probes on chips but also a versatile and powerful tool for the investigation of the binding activity and electron-transfer kinetics of cationic analytes on DNA-modified surfaces.

Recent developments in nanoscience research have demonstrated that DNA switches (rationally designed DNA nanostructures) constitute a class of versatile building blocks for the fabrication and assembly of electronic devices and sensors at the nanoscale. Functional DNA sequences and structures such as aptamers, DNAzymes, G-quadruplexes, and i-motifs can be readily prepared in vitro, and subsequently adapted to an electrochemical platform by coupling with redox reporters. The conformational or conduction switching of such electrode-bound DNA modules in response to an external stimulus can then be monitored by conventional voltammetric measurements. In this review, we describe how we are able to design and examine functional DNA switches, particularly those systems that utilize electrochemical signaling. We also discuss different available options for labeling functional DNA with redox reporters, and comment on the function-oriented signaling pathways.

Electrochemical reactions at the interfaces of immiscible electrolyte solutions (ITIES) are of fundamental importance in the fields of chemical, biological and pharmaceutical sciences. Four-electrode cell setup, scanning electrochemical microscopy (SECM) and thin-film voltammetry are the three most frequently used methods for studying the electrochemical processes at these interfaces. The principle, experimental design, advantages and challenges of the three methods are described and compared. The thin-film voltammetry is highlighted for its simplicity in experimental operation and kinetic data analysis. Its versatile analytical applications are discussed in detail, including the study of redox properties of hydrophobic compounds, evaluation of interfacial electron transfer kinetics, synthesis of nanoparticles/nanostructures, and illustration of cross-membrane ion transport phenomena.

Aptamers have proven to be very useful as high-affinity and -specificity molecular recognition elements in analytical sensors of various forms. Herein, we describe a general process for creating an aptamer-based sensor that functions as an analyte-responsive, nano-sized, electronic switch. These sensors can provide an electrochemical readout, by switching through-DNA charge transfer across a DNA three-way junction from "off" to "on" in response to the binding of a target ligand to the sensor's aptamer domain. We detail the general design principles for such sensors, as well as the biochemical charge transfer assays used to identify functional sensors. In these gel electrophoresis-based assays, analyte-responsive conductivity switching is detected conveniently through biochemical experiments that characterize oxidative DNA damage patterns in sequencing PAGE gels.

Recent developments in nanoscience research have demonstrated that DNA switches (rationally designed DNA nanostructures) constitute a class of versatile building blocks for the fabrication and assembly of electronic devices and sensors at the nanoscale. Functional DNA sequences and structures such as aptamers, DNAzymes, G-quadruplexes, and i-motifs can be readily prepared in vitro, and subsequently adapted to an electrochemical platform by coupling with redox reporters. The conformational or conduction switching of such electrode-bound DNA modules in response to an external stimulus can then be monitored by conventional voltammetric measurements. In this review, we describe how we are able to design and examine functional DNA switches, particularly those systems that utilize electrochemical signaling. We also discuss different available options for labeling functional DNA with redox reporters, and comment on the function-oriented signaling pathways.

Robust and uniform n-alkyl monolayers were formed on silicon from the reaction of Grignard regents (n-CnH2n+1MgBr, n = 2, 6, 10, and 15) with hydrogen-terminated Si(111). The capacitive properties of these organic thin films on silicon in contact with aqueous electrolytes were evaluated by electrochemical impedance measurements. In particular, the reciprocal capacitance of the organic thin film modified silicon/aqueous electrolyte interfaces is proportional to the film thickness, which is tunable by simply varying the alkyl chain length. The derived dielectric constant of these organic thin films from the best fit of the reciprocal capacitance vs ellipsometric film thickness plot is e = 3.3 ± 0.6.

Electron transfer (ET) kinetics through alkyl monolayers, formed on n-type Si(111) surface by the direct reaction of alkylmagnesium bromide (n-CnH2n+1MgBr, n=2, 6, 10, and 15) with hydrogen-terminated Si(111), was investigated in acetonitrile (MeCN) with anthraquinone (AQ) as the electrochemical probe. Cyclic voltammetric measurements indicate that the ability of the monolayer to block interfacial electron transfer increases with increasing alkyl chain length. In particular, the voltammetric behavior changes from non-rectifying (i.e., chemically reversible redox couple), to rectifying (i.e., diode-like when the reverse wave is pushed into the gap) with increasing chain length. The dependence of the logarithm of the electron transfer rate constant as a function on the number of carbons in the alkyl chain is not consistent with electron tunneling through the full thickness of the film. In fact, the measured constant, 0.05 ± 0.03 per methylene, is much smaller than the well-established tunneling constant, ∼1.0/CH2 in the closely packed alkanethiol monolayers on gold suggesting permeation of the AQ into the film. Keywords: Electron transfer, Alkyl monolayers, Si(111), Cyclic voltammetry, Distance dependence

The relationship between the physical properties of azobenzene self-assembled monolayers (SAMs) on gold surfaces (CnH2n+1OφNNφC(O)NHCmH2mSAu, abbreviated as CnAzoCmS−Au) and the structural nature of the parent alkanethiol molecules was investigated by electrochemical in-situ Fourier transform infrared reflection/absorption spectroscopy (FTIR−RAS). The 2H+, 2e- redox behavior from azobenzene to hydrazobenzene in CnAzoCmS−Au monolayers was confirmed, as reported in our recent paper (Langmuir 1998, 14, 619−624). The in-situ spectra showed unambiguously that the molecular orientation and electrochemical stability of azobenzene SAMs are highly related to the structural nature of the azobenzene alkanethiols. In particular, the longer the alkyl chains in the azobenzene alkanethiols, the more stable and closely packed are the monolayers. A potential-induced orientational switching of the terminal ethoxy group was observed for C2AzoC4S−Au, which is different from the intermolecular hydrogen-bonding “fixed” alkyl c...

A silver island film was thermally deposited on the top of an azobenzene self-assembled monolayer on gold, yielding a SERS-active system possessing a “sandwiched” structure of Ag|R1−Azo−R2S−|Au. For the first time, the positions of the Azo groups were controlled by using azobenzenealkanethiols with different terminal groups (R1) or interchain spacers (R2), to clarify the relationship between the SERS effect and the structural nature of the system. As will be shown, increases in distances of Azo groups from the gold substrate and from the silver film both cause the enhancement factor to decay exponentially, indicating that the enhancement correlates to both the gold substrate underneath and the silver islands above.

Two methods, Langmuir–Blodgett (LB) and self-assembly (SA), were used in the present work to fabricate certain interfacial structures for the study of long-range electron transfer. We have successfully constructed two different `bridges' for the interfacial electron transfer in azobenzene LB/SAM composite bilayer systems. One has ionic bonding character, while the other has face-to-face hydrogen bonding at the interface of an ABD (4-octyl-4′-(3-carboxy-trimethylene-oxy)-azobenzene) LB monolayer and the underlying aminothiol SAM. These structures have been confirmed by using RA-FTIR and contact angle titration. Furthermore, the apparent electron transfer rate across the ionic bonding `bridge' is much more rapid than in the hydrogen-bonded case, indicating that the interfacial structure plays a crucial role in the long-range electron transfer kinetics.

The redox behavior of the self-assembled monolayer of 4-ethoxy-4‘-((N-(10‘‘-mercaptodecyl)amino)carbonyl)azobenzene (CH3CH2O−Ph−NN−Ph−C(O)NH(CH2)10−SH, abbreviated as C2AzoC10SH below) on a gold electrode surface has been investigated by electrochemical in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The bands observed in the potential dependent IRRAS spectra corresponded well to the reduction/oxidation process of the azobenzene group in the monolayer. No bands were found in the in-situ FT-IRRAS spectra for the C−H stretch of the methylene group and the N−H stretch as well as the CO stretch of the amide moiety, suggesting that no orientation change occurs in the polymethylene chain of the monolayer during the redox reaction of the azobenzene group. The band intensity observed in the in-situ IRRAS spectra depends on the reaction time, especially in the reduction process of the azobenzene monolayer. Our results provided direct evidence for a slower reduction of azobenzene t...

The photochromism and electrochemical behavior of a Langmuir-Blodgett (LB) monolayer formed by a novel crown-ether-derived azobenzene derivative (CAD) are reported. The large crown-ether functionally was introduced into the CAD molecule as a spacer to provide enough free volume for azobenzene trans-cis isomerization and also as the hydrophilic head group for LB film fabrication. The CAD molecules formed a stable monolayer at the air/water interface which could also be transferred well to solid substrates. The photochemical conversion rate from trans to cis isomers in this azobenzene monolayer reached 76 ± 4%, showing a great increase compared with that in a simple long-chain fatty acid azobenzene LB film (30%). It is only a little smaller than the value obtained in solution (86%). The two isomers in the LB films showed significantly different electrochemical responses, with the cis isomer being more easily reduced than the trans isomer. The electron transfer rate constant k s was also evaluated. The present studies confirmed that the incorporation of a large group can greatly improve the photochromism of azobenzene in condensed films and demonstrated the great potential of molecular design for controlling the functionality of monolayer assemblies.