A novel "off-on" ratiometric fluorescent aptasensor was established for adenosine detection based on fluorescence resonance energy transfer (FRET) between CdS QDs, DNA QDs as donor and graphene oxide (GO) as acceptor. Aminoriched DNA QDs covalently bonded to the carboxyl group on the edge of the GO, and with the absorption of the TGA-modified CdS QDs with aptamer (CdS QDs-apt) onto the GO surface via the 7C-7C stacking interaction. The fluorescence of both CdS QDs and DNA QDs were efficiently quenched due to FRET (turn off). When adenosine was present, the specific binding of the aptamer to the target preferentially that released the CdS QDs-apt from GO. The process would inhibit the FRET which contribute to the fluorescence of CdS QDs-apt recovery again (turn on), while the fluorescence intensity of DNA QDs only slightly altered and acted as the reference signal. Thus, a novel "off-on" ratiometric fluorescent aptasensor for adenosine detection was constructed accordingly. There was a good linearity relationship between the ratio of the FL intensity (F595 nm/F464 nm) and the concentration of adenosine in the range of 20.00-180.0 nmol/L with a detection limit of 1.3 nmol/L (S/N = 3, n = 9). Importantly, the feasibility of the developed aptasensor for selective detection of adenosine in serum and urine samples with satisfactory results. The recoveries were observed to be 97.04-100.2 %.

Based on DNA bio-dots-induced aggregation of gold nanoparticles (AuNPs), a methionine (Met) photoelectrochemical (PEC) sensor with CS–GSH–CuNCs/TiO2 NPs as the photoelectric conversion element and AuNPs as the specific recognition element was constructed. First, a TiO2 NPs/ITO electrode and CS–GSH–CuNCs were prepared, and then the CS–GSH–CuNCs/TiO2 NPs/ITO photosensitive electrode was obtained by self-assembly. Next, DNA bio-dots were modified to the upper surface of the electrode using a coupling reaction to assemble the DNA bio-dots/CS–GSH–CuNCs/TiO2 NPs electrode. Amino-rich DNA bio-dots were used to induce the aggregation of AuNPs on the electrode surface via Au–N interactions and prepare the AuNPs/DNA bio-dots/CS–GSH–CuNCs/TiO2 NPs electrode. Due to the fluorescence resonance energy transfer (FRET) between CS–GSH–CuNCs and AuNPs, the complexation chance of electron-hole (e−-h+) pair in CS–GSH–CuNCs increased, which, in turn, led to a decrease in photocurrent intensity. When Met was present, AuNPs aggregated on the electrode surface were shed and bound to Met since the Au–S interaction is stronger than the Au–N interaction, resulting in the recovery of the photocurrent signal. Under optimal conditions, the photocurrent intensity of the PEC sensor showed good linearity with the logarithm of Met concentration in the range of 25.0 nmol/L–10.0 μmol/L with the limit of detection (LOD) of 5.1 nmol/L (S/N = 3, n = 10).

The turn-on fluorescence signal mechanism for detection of GSH.

An ascorbic acid (AA) sensor was constructed based on the fluorescence resonance energy transfer (FRET) between CdS quantum dots (CdS QDs) and polydopamine (PDA) to detect trace AA sensitively. FRET occurred due to the broad absorption spectrum of PDA completely overlapped with the narrow emission spectrum of CdS QDs. The fluorescence of CdS QDs was quenched and in the “off” state. When AA was present, the conversion of DA to PDA was hindered and the FRET disappeared, resulting in the fluorescence of CdS QDs in an “on” state. Importantly, the degree of fluorescence recovery of CdS QDs displayed a desirable linear correlation with the concentration of AA in the range of 5.0–100.0 μmol/L, the linear equation is y=0.0119cAA+0.3113, and the detection limit is 1.16 μmol/L (S/N = 3, n = 9). There was almost no interference with common amino acid, glucose and biological sulfhydryl small molecules to AA. Trace amount of AA in vitamin C tablets were determined and satisfactory results were obtained; the recoveries were observed to be 98.01–100.7%.

An off-on fluorescence aptasensor was developed for trace thrombin detection based on fluorescence resonance energy transfer (FRET) between CdS QDs and gold nanoparticles (AuNPs). Using DNA pairwise hybridization of the aptamer to the complementary DNA (cDNA), the CdS QDs (energy donor) were tightly coupled to the AuNPs (energy acceptor), resulting in the occurrence of FRET and there was a dramatic fluorescence quenching of CdS QDs (turn off). When the thrombin was added to the fluorescence aptasensor, the specific binding of the aptamer to the target formed a G-quadruplex that caused the AuNPs receptor to detach and the DNA duplex to be disassembled. The process would inhibit the FRET which contribute to the recovery of fluorescence (turn on) and an "off-on" fluorescence aptasensor for thrombin detection was constructed accordingly. Under optimal conditions, the fluorescence recovery showed good linearity with the concentration of thrombin in the range of 1.35-54.0 nmol L-1, and the detection limit was 0.38 nmol L-1 (S/N = 3, n = 9). Importantly, the fluorescence aptasensor presented excellent specificity for thrombin, and was successfully applied to the quantitative determination of thrombin in real serum with satisfactory recoveries of 98.60-102.2%.

A photoelectrochemical (PEC) sensor combining chitosan-coated and glutathione-protected copper nanocluster (CS-GSH-CuNCs) with peroxidase-like activity and xanthine oxidase (XAO) was developed for rapid detection of xanthine. Firstly, CS-GSH-CuNCs was prepared by glutathione (GSH) reduction using chitosan (CS) as protective agent, and then was modified on titanium dioxide nanoparticles (TiO2 NPs)/ITO electrode which was prepared by sintering titanium dioxide (TiO2) on the ITO electrode surface by high temperature calcination. After excited by light at 350 nm, the photoinduced electrons from the lowest unoccupied molecular orbital (LUMO) of CS-GSH-CuNCs were transferred to the conduction band of TiO2, which resulted in spatial separation of electron-hole and enhancement of photocurrent signal. XAO oxidizes xanthine to produce hydrogen peroxide (H2O2). Part of the photoinduced electrons from CS-GSH-CuNCs transferred and catalyzed the reduction of H2O2, resulting in the reduction of the photocurrent. Under optimal conditions, the PEC sensor exhibits good sensitivity and reproducibility with the limit of detection (LOD) of 6.6 nmol . L-1 and a relative standard deviation of 3.5 % for 10 replicate detections of 1.00 mu mol . L-1 xanthine and the linear range of 0.04-90.0 mu mol . L-1 for xanthine. Furthermore, the PEC sensor presents nice selectivity owing to its enzyme-like activity and was successfully applied to human urine, indicating its great potential for real sample analysis.

A dual recognition system with a fluorescence quenching of quantum dots (QDs) and specific recognition of molecularly imprinted polymer (MIP) for the detection of chloramphenicol (CAP) was constructed. MIP@SiO2@QDs was prepared by reverse microemulsion method with 3-aminopropyltriethoxysilane (APTS), tetraethyl orthosilicate (TEOS) and QDs being used as the functional monomer, cross-linker and signal sources, respectively. MIP can specifically recognize CAP, and the fluorescence of QDs can be quenched by CAP due to the photo-induced electron transfer reaction between CAP and QDs. Thus, a method for the trace detection of CAP based on MIP@SiO2@QDs fluorescence quenching was established. The fluorescence quenching efficiency of MIP@SiO2@QDs displayed a desirable linear response to the concentration of CAP in the range of 1.00~4.00 × 102 μmol × L−1, and the limit of detection was 0.35 μmol × L−1 (3σ, n = 9). Importantly, MIP@SiO2@QDs presented good detection selectivity owing to specific recognition for CAP, and was successfully applied to quantify CAP in lake water with the recovery ranging 102.0~104.0%, suggesting this method has the promising potential for the on-site detection of CAP in environmental waters.

构建了一种以硫化镉量子点(CdS QDs)和二氧化钛纳米颗粒(TiO2 NPs)为光敏材料的四环素光电化学适体传感器.以烧结和自组装方式将TiO2 NPs,CdS QDs修饰于ITO电极表面,制得CdS QDs/TiO2 NPs/ITO光敏电极.由于CdS QDs具有比TiO2 NPs更高的导带能级,当CdS QDs吸收420 nm的可见光被激发时,产生的光生电子(e-)将转入TiO2 NPs的导带能级,而空穴(h+)位于CdS QDs的价带能级,实现了e--h+对的分离,提高了光电转换效率.利用偶联反应将TC适体修饰于CdS QDs/TiO2 NPs/ITO光敏电极表面,适体分子所产生的空间位阻效应将抑制光电转换效率,导致光电流信号降低;而TC可与其适体发生特异性结合被电极上的适体捕获,使TC适体从光敏电极表面脱落,光电流信号因此得以恢复.结果表明:该方法在浓度为0.01～15.00μmol·L-1时呈良好的线性关系;检出限为4.2 nmol·L-1(S/N=3);加标回收率为97.6％～104.7％;且对土霉素、金霉素、氯霉素、氨苄西林等与TC相似抗生素具有较好的抗干扰能力.表明TC PEC适体传感器在检测牛奶和兽药等实际样品中的TC有较好的前景.

Kanamycin (Kana) is widely used as a veterinary medicine and its abuse causes a serious threat to human health, raising the urgent demand for detection of residual Kana in animal-derived food with high specificity and sensitivity. Here, we developed a photoelectrochemical (PEC) biosensor for rapid quantification of Kana, with lead sulfide quantum dots/titanium dioxide nanoparticles (PbS QDs/TiO2 NPs) as a photosensitive composite, a Kana-specific DNA aptamer as a functional sensor, and ruthenium(III) hexaammine (Ru(NH3)63+) as a signal booster. To prepare the PEC aptasensor, TiO2 NPs, PbS QDs, and polyethyleneimine (PEI) were respectively used to modify the indium tin oxide electrode, and then the amine-terminated aptamer probe was connected to the PEI via glutaraldehyde. Finally, Ru(NH3)63+ was attached on the surface of the aptamer to increase the photocurrent intensity. When Kana binds competitively with Ru(NH3)63+ to the aptamer immobilized on the surface of the aptasensor, Ru(NH3)63+ will be released from the aptamer, resulting in a decrease of the photocurrent signal. This PEC aptasensor exhibits a good linear relationship between the photocurrent shift and the logarithm of Kana concentration within the range of 1.0-300.0 nmol L-1, and the detection limit is 0.161 nmol L-1. Importantly, the PEC aptasensor presented good detection selectivity owing to specific interaction with Kana and was successfully implemented to quantify Kana in honey and milk, suggesting that the PEC aptasensor has the potential of rapid detection of residual Kana in animal-derived foods.

A tetraphenylporphyrin (TPP) doped PFBT polymer quantum dots (TP-Pdots) were synthesized via the reprecipitation method for photoelectrochemical (PEC) aptasensor detection of tetracycline (TC). The TP-Pdots exhibit a superior cathode photocurrent signal. TP-Pdots increase the separation of photo-generated charges, and improve photocurrent conversion efficiency, resulting in enhanced photocurrent response. The aptamer was used as a recognition element and TP-Pdots as a photoactive material to prepare a PEC aptasensor for sensitivity detection of TC. The PEC aptasensor was constructed by immobilizing TP-Pdots on ITO electrode and combining it with aptamer by EDC coupling. After the TC reacts specifically with the aptamer, causing the aptamer to fall off from the TP-Pdots /ITO electrodes surface and the photocurrent intensity restored. This PEC aptamer sensor possesses a wide linear range from 1.0 nmol.l(-1) to 1.0 x 10(4) nmol.l(-1) with the detection limit of 0.26 nmol.l(-1). Meanwhile, the PEC aptasensor was successfully used for the detection of tetracycline in honey. (C) 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.

Precise dispensing of nanoliter droplets is necessary for the development of sensitive and accurate assays, especially when the availability of the source solution is limited. Conventional approaches are limited by imprecise positioning, large shear forces, surface tension effects, and high costs. To address the need for precise and economical dispensing of nanoliter volumes, we developed a new approach where the dispensed volume is dependent on the size and shape of defined surface features, thus freeing the dispensing process from pumps and fine gauge needles requiring accurate positioning. The surface we fabricated, called a nanoliter droplet virtual well microplate (nVWP) achieves high precision dispensing (better than ± 0.5 nL or ±1.6% at 32 nL) of 20-40 nL droplets using a small source drop (3-10 µL) on isolated hydrophilic glass pedestals (500 µm on a side) bonded to arrays of polydimethylsiloxane conical posts. The sharp 90o edge of the glass pedestal pins the solid-liquid-vapor triple contact line (TCL), averting the wetting of the glass sidewalls while preventing the fluid from receding from the edge. This edge creates a sufficiently large energy barrier that microliter water droplets can be poised on the glass pedestals exhibiting contact angles greater > 150o. This approach relieves the stringent mechanical alignment tolerances required for conventional dispensing techniques, shifting the control of dispensed volume to the area circumscribed by the glass edge. The effects of glass surface chemistry and dispense velocity on droplet volume were studied using optical microscopy and high-speed video. Functionalization of the glass pedestal surface enabled the selective adsorption of specific peptides and proteins from synthetic and natural biomolecule mixtures such as venom. We further demonstrate how the nVWP dispensing platform can be used for a variety of assays including sensitive detection of proteins and peptides by fluorescence microscopy or MALDI-TOF.