New imidazole‐functionalized polyfluorene derivatives: convenient postfunctional syntheses,sensitive probes for metal ions and cyanide,and adjustable output signals with diversified fluorescence color

Based on our previous work on the sensitive and selective conjugated fluorescent polymeric sensors toward cyanide, 2,1,3‐benzothiadiazole and 4,7‐bis(thiophen‐2‐yl)‐2,1,3‐benzothiadiazole were incorporated into the polyfluorene backbone to yield three new polymers bearing imidazole moieties in the side chains, with different fluorescence color. The fluorescence could be turned off by Cu²⁺ ions and then recovered on addition of cyanide, making them good cyanide sensors with the detection limit down to 1.9 μM. Moreover, by fully understanding this “turn off–turn on” strategy and using the cooperation of two polymers with different fluorescence color, the emission color of the mixture system of one of the imidazole‐containing polymers and one from the corresponding polymers without imidazole ones, could be adjusted by the concentrations of the added copper and cyanide ions, leading to the output fluorescent signals diversity. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Visual detection of lead(II) using a label-free DNA-based sensor and its immobilization within a monolithic hydrogel

Lead is highly toxic and its detection has attracted a lot of research interests. In recent years, DNA has been used for Pb(2+) recognition and many fluorescent sensors with low to sub-nM detection limits have been reported. These figures of merit were typically measured using a spectrophotometer that can detect nM DNA with a high signal-to-noise ratio. For visual detection, however, μM DNA or dye was required, making it difficult to detect low nM targets. We recently achieved a visual sensitivity of 10 nM Hg(2+) by immobilizing a DNA probe in a hydrogel. This was made possible because the gel was able to actively adsorb Hg(2+). In this work, we aim to test whether this method can be extended to the detection of Pb(2+). First, a new Pb(2+) sensor was designed based on a guanine-rich DNA and DNA binding dyes such as thiazole orange and SYBR Green I. The free DNA showed a detection limit of 8 nM Pb(2+) using 40 nM DNA. For visual detection in solution with 1 μM of the DNA probe, however, ～300 nM Pb(2+) was required. After immobilization in a monolithic polyacrylamide hydrogel, even 20 nM Pb(2+) could be visually detected with a sample volume of 50 mL. Therefore, sensitive detection without signal amplification was achieved. Finally, we demonstrated simultaneous detection of both Hg(2+) and Pb(2+) in the same water sample with shape encoded hydrogel sensors.     

Selective colorimetric sensing of mercury(II) using turn off-turn on mechanism from riboflavin stabilized silver nanoparticles in aqueous medium

A simple Hg(2+) sensor has been developed using the vitamin B2 (riboflavin) stabilized Ag nanoparticle via a "turn off"-"turn on" mechanism; both the colour and photoluminescence properties of the riboflavin solution are used as sensitizing tools showing a sensitivity up to 5 nM Hg(2+) concentration.     

Label‐free Electrochemiluminescent Enantioselective Sensor for Distinguishing between Chiral Metallosupramolecular Complexes

Chiral molecular recognition of DNA is important for rational drug design and for developing structural probes of DNA conformation. Developing a convenient and inexpensive assay for sensitive and selective identification of DNA‐specific binding compounds with rapid, easy manipulation is in ever‐increasing demand. Here, we present a “turn‐on” and label‐free electrochemiluminescent (ECL) biosensor for distinguishing chiral metallosupramolecular complexes based on DNA three‐way junction formation selectively induced by the analyte. The fabricated ECL sensor shows excellent performance in the chiral discrimination of two enantiomers with an enantioselective recognition ratio of up to 4.4. More importantly, as a “turn‐on” detection system, the ECL chiral sensor does not suffer from false positives and limited signal range of “signal‐off” systems. Therefore, this concept may provide a new insight into the design of efficient sensors for distinguishing chiral molecules and for investigating the interactions between DNA and small molecules.     

Comparison of Insulin Determination on NiNPs/chitosan‐ MWCNTs and NiONPs/chitosan‐MWCNTs Modified Pencil Graphite Electrode

The rising amount of patients suffering for diabetes mellitus increases the requirements for effective insulin sensors. Carbon materials are a suitable choice for the development of insulin sensors due to their electrochemical characteristics. Pencil graphite electrodes (PGE) represent the trade‐off between price and excellent conductive properties. The modification of PGE by NiO and Ni nanoparticles fixed by chitosan results in surface area enlargement and improved electrocatalytic properties. This paper is focused on the comparison of different properties of Ni and NiO nanoparticles and their effect on redox reaction mechanism of insulin and detection characteristics. The electrode modified by Ni nanoparticles displays linear range of 1 μM–5 μM (R² 0.80), limit of detection (LOD) of 4.34 μM and sensitivity of 0.12 μA/μM. On the other hand, the electrode modified by NiO nanoparticles displays enhanced electrochemical characteristics such as linear range of 0.05 μM–5 μM (R² 0.99), limit of detection of 260 nM and sensitivity of 0.64 μA/μM. These properties make the NiO nanoparticles modified PGE the appropriate candidate for insulin determination.     

Incorporation of extra amino acids in peptide recognition probe to improve specificity and selectivity of an electrochemical peptide-based sensor

We investigated the effect of incorporating extra amino acids (AA) at the n-terminus of the thiolated and methylene blue-modified peptide probe on both specificity and selectivity of an electrochemical peptide-based (E-PB) HIV sensor. The addition of a flexible (SG)₃ hexapeptide is, in particular, useful in improving sensor selectivity, whereas the addition of a highly hydrophilic (EK)₃ hexapeptide has shown to be effective in enhancing sensor specificity. Overall, both E-PB sensors fabricated using peptide probes with the added AA (SG-EAA and EK-EAA) showed better specificity and selectivity, especially when compared to the sensor fabricated using a peptide probe without the extra AA (EAA). For example, the selectivity factor recorded in the 50% saliva was ∼2.5 for the EAA sensor, whereas the selectivity factor was 7.8 for both the SG-EAA and EK-EAA sensors. Other sensor properties such as the limit of detection and dynamic range were minimally affected by the addition of the six AA sequence. The limit of detection was 0.5 nM for the EAA sensor and 1 nM for both SG-EAA and EK-EAA sensors. The saturation target concentration was ∼200 nM for all three sensors. Unlike previously reported E-PB HIV sensors, the peptide probe functions as both the recognition element and antifouling passivating agent; this modification eliminates the need to include an additional antifouling diluent, which simplifies the sensor design and fabrication protocol.     

Phenoxazine‐based Near‐infrared Fluorescent Probes for the Specific Detection of Copper (II) Ions in Living Cells

Abstract : It is well known that copper ions play a critical role in various physiological processes. However, a variety of human diseases are tightly correlated with copper overload. Although there are numerous fluorescent probes capable of detecting copper ions, most of them are “turn‐off” probes owing to copper (II) ions fluorescence quenching effect, resulting in poor sensitivity. Herein, a novel “turn‐on” near‐infrared (NIR) fluorescent probe PZ‐N based on phenoxazine was designed and synthesized for the selective detection of copper (II) ions (Cu²⁺). Upon the addition of Cu²⁺, the probe could quickly react with Cu²⁺ and emit strong fluorescence, along with colour change from colourless to obvious blue. Moreover, the probe PZ‐N showed good water solubility, high selectivity, and excellent sensitivity with low limit of detection (1.93 nM) towards copper (II) ions. More importantly, PZ‐N was capable of effectively detecting Cu²⁺ in living cells.     

The Split Primer Ligation‐triggered 8‐17 DNAzyme Assisted Cascade Rolling Circle Amplification for High Specific Detection of Liver Cancer‐involved mRNAs: TK1 and c‐myc

Simultaneous detection of various intracellular biomarkers is promising for early diagnosis and treatment of cancer. Herein, we develop a novel method for high specific and ultrasensitive detection of liver cancer cell‐involved mRNAs: TK1 and c‐myc based on the split primer ligation‐triggered 8‐17 DNAzyme assisted cascade rolling circle amplification. Only two targets exist simultaneously, can trigger the rolling circle amplification to improve the accuracy and sensitivity. Meanwhile, an electrochemical molecular beacon, based on the host‐guest recognition between ferrocene groups and cucurbit urils [7] (CB[7]/Fc‐MB), is used to cause a “turn‐off” electrochemical signal which is decreased by disrupting its hairpin structure. Under the optimal conditions, the detection limit of TK1 and c‐myc mRNA is as low as 0.06 nM. Moreover, this method can be used to detect the TK1 and c‐myc mRNA in HepG2 cells and distinguish between cancer cells and their normal cells, proving that the method has the potential to detect the variation of biomarkers in vivo.     

Cytidine-stabilized gold nanocluster as a fluorescence turn-on and turn-off probe for dual functional detection of Ag and Hg

In this study, we have developed a label-free, dual functional detection strategy for highly selective and sensitive determination of aqueous Ag⁺ and Hg²⁺ by using cytidine stabilized Au NCs and AuAg NCs as fluorescent turn-on and turn off probes, respectively. The Au NCs and AuAg NCs showed a remarkably rapid response and high selectivity for Ag⁺ and Hg²⁺ over other metal ions, and relevant detection limit of Ag⁺ and Hg²⁺ is ca. 10 nM and 30 nM, respectively. Importantly, the fluorescence enhanced Au NCs by doping Ag⁺ can be conveniently reusable for the detection of Hg²⁺ based on the corresponding fluorescence quenching. The sensing mechanism was based on the high-affinity metallophilic Hg²⁺–Ag⁺ interaction, which effectively quenched the fluorescence of AuAg NCs. Furthermore, these fluorescent nanoprobes could be readily applied to Ag⁺ and Hg²⁺ detection in environmental water samples, indicating their possibility to be utilized as a convenient, dual functional, rapid response, and label-free fluorescence sensor for related environmental and health monitoring.     

Highly sensitive and selective colorimetric sensors for uranyl (UO2(2+)): development and comparison of labeled and label-free DNAzyme-gold nanoparticle systems

Colorimetric uranium sensors based on uranyl (UO2(2+)) specific DNAzyme and gold nanoparticles (AuNP) have been developed and demonstrated using both labeled and label-free methods. In the labeled method, a uranyl-specific DNAzyme was attached to AuNP, forming purple aggregates. The presence of uranyl induced disassembly of the DNAzyme functionalized AuNP aggregates, resulting in red individual AuNPs. Once assembled, such a "turn-on" sensor is highly stable, works in a single step at room temperature, and has a detection limit of 50 nM after 30 min of reaction time. The label-free method, on the other hand, utilizes the different adsorption properties of single-stranded and double-stranded DNA on AuNPs, which affects the stability of AuNPs in the presence of NaCl. The presence of uranyl resulted in cleavage of substrate by DNAzyme, releasing a single stranded DNA that can be adsorbed on AuNPs and protect them from aggregation. Taking advantage of this phenomenon, a "turn-off" sensor was developed, which is easy to control through reaction quenching and has 1 nM detection limit after 6 min of reaction at room temperature. Both sensors have excellent selectivity over other metal ions and have detection limits below the maximum contamination level of 130 nM for UO2(2+) in drinking water defined by the U.S. Environmental Protection Agency (EPA). This study represents the first direct systematic comparison of these two types of sensor methods using the same DNAzyme and AuNPs, making it possible to reveal advantages, disadvantages, versatility, limitations, and potential applications of each method. The results obtained not only allow practical sensing application for uranyl but also serve as a guide for choosing different methods for designing colorimetric sensors for other targets.     

Ultra-sensitive hydrazine chemical sensor based on high-aspect-ratio ZnO nanowires

High-aspect-ratio ZnO nanowires based ultra-sensitive hydrazine amperometric sensor has been fabricated which showed a high and reproducible sensitivity of 12.76 μA cm⁻² nM⁻¹, detection limit, based on S/N ratio, 84.7 nM, response time less than 5 s, linear range from 500 to 1200 nM and correlation coefficient of R = 0.9989. This is the first report in which such a very high-sensitivity and low detection limit has been achieved for the hydrazine sensors by using ZnO nanostructures modified electrodes. Therefore, this work opens a way to utilize simply grown ZnO nanostructures as an efficient electron mediator to fabricate efficient hydrazine sensors.     

Effect of redox label tether length and flexibility on sensor performance of displacement-based electrochemical DNA sensors

This article summarizes the sensor performance of four electrochemical DNA sensors that exploit the recently developed displacement-replacement sensing motif. In the absence of the target, the capture probe is partially hybridized to the signaling probe at the distal end, positioning the redox label, methylene blue (MB), away from the electrode. In the presence of the target, the MB-modified signaling probe is released; one type of probe is capable of assuming a stem-loop probe (SLP) conformation, whereas the other type adopts a linear probe (LP) conformation. Independent of the sensor architecture, all four sensors showed “signal-on” sensor behavior. Unlike the previous report, here we focused on elucidating the effect of the redox label tether length and flexibility on sensor sensitivity, specificity, selectivity, and reusability. For both SLP and LP sensors, the limit of detection was 10 pM for sensors fabricated using a signaling probe with three extra thymine (T3) bases linked to the MB label. A limit of detection of 100 pM was determined for sensors fabricated using a signaling probe with five extra thymine (T5) bases. The linear dynamic range was between 10 pM and 100 nM for the T3 sensors, and between 100 pM and 100 nM for the T5 sensors. When compared to the LP sensors, the SLP sensors showed higher signal enhancement in the presence of the full-complement target. More importantly, the SLP-T5 sensor was found to be highly specific; it is capable of discriminating between the full complement and single-base mismatch targets even when employed in undiluted blood serum. Overall, these results highlight the advantages of using oligo-T(s) as a tunable linker to control flexibility of the tethered redox label, so as to achieve the desired sensor response.     

“Oxidative etching-aggregation” of silver nanoparticles by melamine and electron acceptors: An innovative route toward ultrasensitive and versatile functional colorimetric sensors

An innovative and versatile functional colorimetric sensor for melamine (MA) and H₂O₂ was developed with simplicity, excellent selectivity and ultrasensitivity. The detection mechanism was based on the “oxidative etching-aggregation” of silver nanoparticles (AgNPs) by the cooperation effect of MA and electron acceptors such as H₂O₂, ozone or Fe(NO₃)₃. The detection limits of this method for MA could reach as low as 0.08 nM, 0.16 nM and 3 nM when H₂O₂, ozone or Fe(NO₃)₃ was used as an electron acceptor, respectively. When using H₂O₂ as a typical electron acceptor, the method enabled the detection of H₂O₂ with a detection limit of 0.2 nM. This proposed method offered a new way to design MA and H₂O₂ sensors and might be easily extended to detect other nucleophilic reagents and electron acceptors based on colorimetric sensors.     

PCB-based integration of electrochemiluminescence detection for microfluidic systems

This communication presents an instrumental development based on the printed circuit board (PCB) technology to integrate electrochemiluminescence (ECL) analysis in microfluidic systems. PCB gold macro- (10 mm2) and micro- (0.09 mm2) electrodes and two ECL microfluidic devices are designed, fabricated and tested via luminol ECL detection. Potential modulation is performed between 0.7 and 0 V vs. Ag/AgCl for luminol oxidation, thus giving rise to on/off ECL responses in the presence of hydrogen peroxide. Synchronous detection is adopted to allow weak ECL signal recovery at a very low signal-to-noise ratio (SNR). The detection limit obtained with the two ECL microfluidic devices is 50 nM and 100 nM H2O2 for macroelectrodes and microelectrodes, respectively.     

Heterogeneous and Homogeneous Aptamer‐Based Electrochemical Sensors for Thrombin

Nanomolar concentrations of thrombin were electrochemically monitored using heterogeneous switch‐on and homogeneous switch‐off approaches that incorporated ferrocenyl aptamers. For the first time, the heterogeneous approach was coupled to a glucose/glucose oxidase (GOx) amplification‐regeneration system which increased its sensitivity by 2 folds with detection limits of 4.3 nM and 2.5 nM in the absence and presence of glucose/GOx, respectively. We also present a new homogeneous system involving the ferrocenyl aptamer binding thrombin in solution causing a significant decrease in its diffusion coefficient. Thus the ferrocene anodic current decreased at an unmodified gold electrode with detection limit of 3.9 nM and 12 times larger linear range than the heterogeneous method.     

Cylindrical sensor geometry for absorbance-based fiber-optic ammonia sensors

A novel cylindrical shape geometry is proposed for fiber-optic ammonia sensors based on chromophoric indicator dyes. Sensors are constructed by trapping the internal indicator solution inside a short segment of a gas-permeable tube. Fiber-optic probes are used to supply incident radiation and to collect light that transverses through the internal solution. This cylindrical sensor geometry provides large optical path lengths which permits the use of chromophoric indicator dyes. Unlike the conventional distal tip geometry, the diffusion path is independent of the optical path which results in short response times coupled with high sensitivity and low limits of detection. Our experiments indicate that stray radiation is negligible for this sensor design, and that the optical path length essentially equals the distance between the fiber-optic probes. Sensors constructed with Bromothymol Blue as the indicator dye are evaluated. As part of this evaluation, three different modes of operation are tested. The best analytical performance obtained when a single discrete aliquot of the internal solution is used. Steady-state responses are achieved within 13-16 min for 200 nM levels of ammonia from sensors with limits of detection ranging from 150 to 20 nM.     

Recent advances in electrochemical sensors for antibiotics and their applications

The abuse of antibiotics will cause an increase of drug-resistant strains and environmental pollution,which in turn will affect human health.Therefore,it is important to develop effective detection techniques to determine the level of antibiotics contamination in various fields.Compared with traditional detection methods,electrochemical sensors have received extensive attention due to their advantages such as high sensitivity,low detection limit,and good selectivity.In this mini review,we summarized the latest developments and new trends in electrochemical sensors for antibiotics.Here,modification methods and materials of electrode are discussed.We also pay more attention to the practical applications of antibiotics electrochemical sensors in different fields.In addition,the existing problems and the future challenges ahead have been proposed.We hope that this review can provide new ideas for the development of electrochemical sensors for antibiotics in the future.     

Naphthalimide-based fluorescent off/on probes for the detection of thiols

Herein, we report the design and synthesis of two naphthalimide-based fluorescent probes. These probes provided high on/off signal ratios and exhibited good selectivity towards thiols over other analytes. Thus, they were identified as good sensors for the detection of thiols both in living cells and in rabbit plasma samples.     

Oligonucleotide-functionalized gold nanoparticles-enhanced QCM-D sensor for mercury(II) ions with high sensitivity and tunable dynamic range

A quartz crystal microbalance with dissipation monitoring (QCM-D) sensor was developed for highly sensitive and specific detection of mercury(II) ions (Hg(2+)) with a tunable dynamic range, using oligonucleotide-functionalized gold nanoparticles (GNPs) for both frequency and dissipation amplification. The fabrication of the sensor employed a 'sandwich-type' strategy, and formation of T-Hg(2+)-T structures in linker DNA reduced the hybridization of the GNPs-tagged DNA on the gold electrode, which could be used as the molecular switch for Hg(2+) sensing. This QCM-D mercury sensor showed a linear response of 10-200 nM, with detection limits of 4 nM and 7 nM for frequency and dissipation measurements, respectively. Moreover, the dynamic range of the sensor could be tuned by simply altering the concentration of linker DNA without designing new sensors in the cases where detection of Hg(2+) at different levels is required. This sensor afforded excellent selectivity toward Hg(2+) compared with other potential coexisting metal ions. The feasibility of the sensor was demonstrated by analyzing Hg(2+)-spiked tap- and lake-water samples with satisfactory recoveries. The proposed approach extended the application of the QCM-D system in metal ions sensing, and could be adopted for the detection of other analytes when complemented with the use of functional DNA structures.     

Label-free biosensors based on aptamer-modified graphene field-effect transistors

A label-free immunosensor based on an aptamer-modified graphene field-effect transistor (G-FET) is demonstrated. Immunoglobulin E (IgE) aptamers with an approximate height of 3 nm were successfully immobilized on a graphene surface, as confirmed by atomic force microscopy. The aptamer-modified G-FET showed selective electrical detection of IgE protein. From the dependence of the drain current variation on the IgE concentration, the dissociation constant was estimated to be 47 nM, indicating good affinity and the potential for G-FETs to be used in biological sensors.