Common Background Signals in Voltammograms of Crystalline Silicon Electrodes are Reversible Silica-Silicon Redox Chemistry at Highly Conductive Surface Sites

The electrochemical reduction of bulk silica, due to its high electrical resistance, is of limited viability, namely, requiring temperatures in excess of 850 °C. By means of electrochemical and electrical measurements in atomic force microscopy, we demonstrate that at a buried interface, where silica has grown on highly conductive Si(110) crystal facets, the silica-silicon conversion becomes reversible at room temperature and accessible within a narrow potential window. We conclude that parasitic signals commonly observed in voltammograms of silicon electrodes originate from silica-silicon redox chemistry. While these findings do not remove the requirement of high temperature toward bulk silica electrochemical reduction, they redefine for silicon the potential window free from parasitic signals and, as such, significantly restrict the conditions where electroanalytical methods can be applied to the study of silicon surface reactivity.

Electrochemical detection of alkaline phosphatase activity through enzyme-catalyzed reaction using aminoferrocene as an electroactive probe

As a nonspecific phosphomonoesterase, alkaline phosphatase (ALP) plays a pivotal role in tissue mineralization and osteogenesis which is an important biomarker for the clinical diagnosis of bone and hepatobiliary diseases. Herein, we described a novel electrochemical method that used aminoferrocene (AFC) as an electroactive probe for the ALP activity detection. In the condition with imidazole and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC), the AFC probe could be directly labeled on single-stranded DNA (ssDNA) by one-step conjugation. Specifically, thiolated ssDNA at 3'-terminals was modified to the electrode surface through Au-S bond. In the condition without ALP, AFC could be labeled on ssDNA by conjugating with phosphate groups. In the presence of ALP, phosphate groups were catalyzed to be removed from the 5'-terminal of ssDNA. The AFC probe cannot be labeled on ssDNA. Thus, the electrochemical detection of ALP activity was achieved. Under optimal conditions, the strategy presented a good linear relationship between current intensity and ALP concentration in the range of 20 to 100 mU/mL with the limit of detection (LOD) of 1.48 mU/mL. More importantly, the approach rendered high selectivity and satisfactory applicability for ALP activity detection. In addition, this method has merits of ease of operation, low cost, and environmental friendliness. Thus, this strategy presents great potential for ALP activity detection in practical applications. An easy, sensitive and reliable strategy was developed for the detection of alkaline phosphatase activity via electrochemical "Signal off".

Intramolecular photoinitiator induced atom transfer radical polymerization for electrochemical DNA detection

A novel electrochemical biosensor was reported for the first time to achieve highly sensitive DNA detection based on photoinduced atom transfer radical polymerization (photoATRP). In this work, PNA was applied as the capture probe to specifically recognize the target DNA (TDNA), and we utilized lung cancer DNA as TDNA. The ATRP initiator was introduced to the electrode surface via phosphate-Zr4+-carboxylate chemistry. PhotoATRP was activated under blue light irradiation based on a photoinitiator I2959, which produced free radicals via homolytic cleavage. Subsequently, Cu2+ was reduced to Cu+ with the assistance of the free radicals, and numerous electroactive probes were grafted onto the electrode surface. Under optimal conditions, the limit of detection (LOD) of this method was 3.16 fM (S/N = 3, R2 = 0.992), and the linear range was from 10 fM to 1.0 nM. More importantly, the preparation process of this biosensor was simple and less laborious with a low background signal, suggesting good potential in practical applications.

Perfluorosulfonic acid polymer based eATRP for ultrasensitive detection of CYFRA21-1 DNA

The sensitive detection of biomarker cytokeratin fragment antigen 21-1 (CYFRA21-1) is crucial for early diagnosis and screening of non-small cell lung cancer (NSCLC). In this work, an electrochemical biosensor based on Nafion-initiated eATRP has been built for ultrasensitive detection of CYFRA21-1 DNA for the first time. Specifically, peptide nucleic acid (PNA) probes are immobilized onto a gold electrode surface and then hybridized with target DNA to form PNA/DNA heteroduplexes for the subsequent attachment of Nafion by the identified carboxyl-Zr4+-phosphoric acid chemistry. Finally, polymer chains are obtained by linking the monomer of ferrocenylmethyl methacrylate to the PNA/MCH/DNA/Zr4+/Nafion probes via eATRP. Under optimized steady-state conditions, the sensor offers a wide current response for CYFRA21-1 DNA from 10-11 to 10-16 M with a detection limit of 6.42 × 10-17 M. The proposed method of using Nafion as the eATRP initiator exhibits high sensitivity, reproducibility and stability and is a promising strategy for early diagnosis of NSCLC.

Ultrasensitive fluorescence detection of sequence-specific DNA via labeling hairpin DNA probes for fluorescein o-acrylate polymers

Sensitive detection of DNA is conducive to enhance the accuracy of diseases diagnosis and risk prediction. In this work, we report the use of activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) as a novel on-chip amplification strategy for the fluorescence detection of DNA. More specifically, the target DNA was captured by the on-chip immobilized hairpin DNA probes. Upon hybridization, exposed 3'-N₃ of the hairpin was used to attach AGET ATRP initiators onto the silicon surface by click chemistry. Then, numerous fluorescent labeling linked to the end of the probes via the formation of long chain polymers of fluorescein o-acrylate, which in turn amplified the fluorescence signal for DNA detection. Under optimal conditions, it showed a good linear range from 100 fM to 1 μM in DNA detection, with the limit of detection as low as 4.3 fM. Moreover, this strategy showed good detection performance in complex real serum samples, the fluorescence intensity of 0.1 nM tDNA in 1% fetal bovine serum samples was 97.6% of that in Tris-EDTA buffer. Based on its high sensitivity, reduced cost and simplicity, the proposed signal amplification strategy displays translational potential in clinical application.

Copper(I)-Catalyzed Click Chemistry as a Tool for the Functionalization of Nanomaterials and the Preparation of Electrochemical (Bio)Sensors

Proper functionalization of electrode surfaces and/or nanomaterials plays a crucial role in the preparation of electrochemical (bio)sensors and their resulting performance. In this context, copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) has been demonstrated to be a powerful strategy due to the high yields achieved, absence of by-products and moderate conditions required both in aqueous medium and under physiological conditions. This particular chemistry offers great potential to functionalize a wide variety of electrode surfaces, nanomaterials, metallophthalocyanines (MPcs) and polymers, thus providing electrochemical platforms with improved electrocatalytic ability and allowing the stable, reproducible and functional integration of a wide range of nanomaterials and/or different biomolecules (enzymes, antibodies, nucleic acids and peptides). Considering the rapid progress in the field, and the potential of this technology, this review paper outlines the unique features imparted by this particular reaction in the development of electrochemical sensors through the discussion of representative examples of the methods mainly reported over the last five years. Special attention has been paid to electrochemical (bio)sensors prepared using nanomaterials and applied to the determination of relevant analytes at different molecular levels. Current challenges and future directions in this field are also briefly pointed out.

A non-spectroscopic optical biosensor for the detection of pathogenic Salmonella Typhimurium based on a stem-loop DNA probe and retro-reflective signaling

The detection of foodborne pathogenic microorganisms is an essential issue in molecular diagnostics. Fluorescence-based assays have been widely utilized in molecular diagnostics because of their ability to detect and measure low analyte concentrations. However, conventional fluorescence-based assays require sophisticated optics systems, such as a specific light source and light filter. To overcome these limitations, we developed an optical sensing system using a retroreflective Janus microparticle (RJP) as a signaling probe. Compared to fluorescent dyes, RJPs have the advantage of not requiring complicated optic systems because they can be observed using visible light without a filter. To confirm that RJPs can be used as a probe for molecular diagnostics, Salmonella was detected using a biotinylated stem-loop DNA probe to capture the target gene DNA and a streptavidin-conjugated RJP (SA-RJP) as the detection molecule. When the target gene DNA was present at the sensing surface where the stem-loop DNA probe was immobilized, the biotinylated stem-loop DNA probe was stretched, exposing biotin, which can react with SA-RJP. Since the amount of exposed biotin increased according to the concentration of the applied target gene DNA, the number of observed RJPs on the sensing surface increased with the concentration of the target gene DNA. Consequently, the concentration of Salmonella could be quantitated by counting the number of observed RJPs. Using this system, Salmonella at concentrations ranging from 0 to 100 nM could be analyzed, with high sensitivity and selectivity, with a limit of detection of 2.48 pM.

Quick Click: The DNA-Templated Ligation of 3'-O-Propargyl- and 5'-Azide-Modified Strands Is as Rapid as and More Selective than Ligase

The copper(I)-mediated azide-alkyne cycloaddition (CuAAC) of 3'-propargyl ether and 5'-azide oligonucleotides is a particularly promising ligation system because it results in triazole linkages that effectively mimic the phosphate-sugar backbone of DNA, leading to unprecedented tolerance of the ligated strands by polymerases. However, for a chemical ligation strategy to be a viable alternative to enzymatic systems, it must be equally as rapid, as discriminating, and as easy to use. We found that the DNA-templated reaction with these modifications was rapid under aerobic conditions, with nearly quantitative conversion in 5 min, resulting in a k_obs value of 1.1 min^-1 , comparable with that measured in an enzymatic ligation system by using the highest commercially available concentration of T4 DNA ligase. Moreover, the CuAAC reaction also exhibited greater selectivity in discriminating C:A or C:T mismatches from the C:G match than that of T4 DNA ligase at 29 °C; a temperature slightly below the perfect nicked duplex dissociation temperature, but above that of the mismatched duplexes. These results suggest that the CuAAC reaction of 3'-propargyl ether and 5'-azide-terminated oligonucleotides represents a complementary alternative to T4 DNA ligase, with similar reaction rates, ease of setup and even enhanced selectivity for certain mismatches.

Kinetics of bulk photo-initiated copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC) polymerizations

Photoinitiation of polymerizations based on the copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction enables spatio-temporal control and the formation of mechanically robust, highly glassy photopolymers. Here, we investigated several critical factors influencing photo-CuAAC polymerization kinetics via systematic variation of reaction conditions such as the physicochemical nature of the monomers; the copper salt and photoinitiator types and concentrations; light intensity; exposure time and solvent content. Real time Fourier transform infrared spectroscopy (FTIR) was used to monitor the polymerization kinetics in situ. Six different di-functional azide monomers and four different tri-functional alkyne monomers containing either aliphatic, aromatic, ether and/or carbamate substituents were synthesized and polymerized. Replacing carbamate structures with ether moieties in the monomers enabled an increase in conversion from 65% to 90% under similar irradiation conditions. The carbamate results in stiffer monomers and higher viscosity mixtures indicating that chain mobility and diffusion are key factors that determine the CuAAC network formation kinetics. Photoinitiation rates were manipulated by altering various aspects of the photo-reduction step; ultimately, a loading above 3 mol% per functional group for both the copper catalyst and the photoinitiator showed little or no rate dependence on concentration while a loading below 3 mol% exhibited 1st order rate dependence. Furthermore, a photoinitiating system consisting of camphorquinone resulted in 60% conversion in the dark after only 1 minute of 75 mW cm-2 light exposure at 400-500 nm, highlighting a unique characteristic of the CuAAC photopolymerization enabled by the combination of the copper(i)'s catalytic lifetime and the nature of the step-growth polymerization.