Sensitive colorimetric visualization of dihydronicotinamide adenine dinucleotide based on anti-aggregation of gold nanoparticles via boronic acid-diol binding

A facile, highly sensitive colorimetric strategy for dihydronicotinamide adenine dinucleotide (NADH) detection is proposed based on anti-aggregation of gold nanoparticles (AuNPs) via boronic acid-diol binding chemistry. The aggregation agent, 4-mercaptophenylboronic acid (MPBA), has specific affinity for AuNPs through Au-S interaction, leading to the aggregation of AuNPs by self-dehydration condensation at a certain concentration, which is responsible for a visible color change of AuNPs from wine red to blue. With the addition of NADH, MPBA would prefer reacting with NADH to form stable borate ester via boronic acid-diol binding dependent on the pH and solvent, revealing an obvious color change from blue to red with increasing the concentration of NADH. The anti-aggregation effect of NADH on AuNPs was seen by the naked eye and monitored by UV-vis extinction spectra. The linear range of the colorimetric sensor for NADH is from 8.0 × 10(-9)M to 8.0 × 10(-6)M, with a low detection limit of 2.0 nM. The as-established colorimetric strategy opened a new avenue for NADH determination.     

A fluorescence resonance energy transfer sensor based on maltose binding protein

A fluorescence resonance energy-transfer (FRET) sensing system for maltose based on E. coli maltose binding protein (MBP) is demonstrated. The FRET donor portion of the sensing system consists of MBP modified with long wavelength-excitable cyanine dyes (Cy3 or Cy3.5). The novel acceptor portion of the sensor consists of beta-cyclodextrin (beta-CD) modified with either the cyanine dye Cy5 or the dark quencher QSY9. Binding of the modified beta-CD to dye-conjugated MBP results in assembly of the FRET complex. Added maltose displaces the beta-CD-dye adduct and disrupts the FRET complex, resulting in a direct change in fluorescence of the donor moiety. In the use of these FRET pairs, MBP dissociation values for maltose were estimated (0.14-2.90 microM). Maltose limits of detection were in the 50-100 nm range.     

A fluorescence enhancement-based sensor using glycosylated metalloporphyrin as a recognition element for levamisole assay

A fluorescence sensor based on the supermolecular recognition by glycosylated metalloporphyrin for levamisole (LEV) assay is reported. For the preparation of a LEV-sensitive active material, 5, 10, 15, 20-tetrakis[2-(2, 3, 4, 6-tetraacetyl-beta-D-glucopyranosyl)-1-O-phenyl] porphyrin and its metal complexes were synthesized and used in an optode membrane prepared by including glycosylated metalloporphyrin in chitosan matrice. The immobilized glycosylated metalloporphyrin is shown to be weakly fluorescent as a result of the inhibiting of the electron tansfer by central metal. The fluorescence enhancement of the metalloporphyrin modified optode membrane by LEV is based on the complexation with the central metal moiety of metalloporphyrin and weakening the inhibiting of the electron tansfer for metalloporphyrin. The glycosylated metalloporphyrin/chitosan optode membrane showed excellent selectivity toward LEV with respect to a number of interferents and exhibited stable response. The calibration graph obtained with the proposed sensor was linear over the range of 1.3x10(-5)-3.5x10(-7)ML(-1), with a detection limit of 3.5x10(-7)ML(-1) for LEV. The prepared sensor is applied for the determination of LEV in pharmaceutical preparations and the results agreed with the values obtained by the pharmacopoeia method.     

The fluorescence sensor for saccharide based on internal conversion.

In this paper, an internal conversion (IC) fluorescence probe N-(o-boronic acid)benzyl-1-naphthylamine (BBNA) was prepared from 1-naphthylamine and 2-formylbenzeneboronic acid. The fluorescence parameters of BBNA were investigated in a variety of solvents. When BBNA interacted with D-fructose in phosphate buffer solution of 30% MeOH, pH 8.21 (v/v), the fluorescence intensity increased and emission maximum red-shifted slightly with increasing D-fructose concentration. In the presence of D-fructose, the fluorescence quantum yield of BBNA increased with increasing solvent polarity, suggesting that internal conversion (IC) occurred with BBNA. The binding force of BBNA with d-fructose was the strongest, and the stability constant (K) of D-fructose was 99.9 mol/L. Therefore, a selective recognition system based on IC was constructed for D-fructose.     

A novel colorimetric fluorescence sensor for Fe3+ based on quinoline Schiff base

A novel fluorescent sensor bearing a quinoline and an anisidine moiety has been developed for highly selective detection of Fe³⁺, which shows photo‐induced electron transfer (PET) behavior induced by Fe³⁺. Binding of Fe³⁺ to the sensor induced the electron of C = N group transfer from quinoline to iron, the result exhibits fluorescent enhancement. With the features of easy synthesis, simple structural skeleton and excellent sensing ability, the newly synthesized chemosensor also applied as a highly selective fluorescent probe in complex samples containing various competitive metal ions. The probe could fulfill various needs in biological and environmental fields.     

A ratiometric fluorescence sensor for Fe3+ based on FRET and PET processes

A rhodamine/coumarin‐based ratiometric fluorescent Fe³⁺ sensor has been designed and synthesized. The sensor exhibits a good response to Fe³⁺ ions with high sensitivity, selectivity and a large shift in the emission spectra (>100 nm), which shows Fe³⁺‐induced FRET OFF–ON and PET ON–OFF behavior. Copyright © 2015 John Wiley & Sons, Ltd.     

Selective recognition of acetate ion based on fluorescence enhancement chemosensor

Fluorescence study of the complexation between uranyl salophen (L) and some common anions in acetonitrile–water (90:10, v/v) solution showed a tendency of L toward acetate ion (AcO^?). The fluorescence enhancement of L is attributed to a 1:1 complex formation between L and acetate ion which was utilized as the basis for the selective detection of AcO^?. The association constant of the 1:1 complex formation of L–AcO^? was calculated as 6.60 × 10⁶. The linear response range of the fluorescent chemosensor covers a AcO^? concentration range of 1.6 × 10^?7 to 2.5 × 10^?5 mol/L, with a detection limit of 2.5 × 10^?8 mol/L. L showed a selective and sensitive fluorescence enhancement response toward acetate ion over I₃^?, NO₃^?, CN^?, CO₃^2?, Br^?, Cl^?, F^?, H₂PO4^? and SO₄^2?, which was attributed to the higher stability of inorganic complex between acetate and L. Copyright © 2012 John Wiley & Sons, Ltd.     

Optical ascorbic acid sensor based on the fluorescence quenching of silver nanoparticles

A sensitive and selective fluorimetric sensor for the assay of ascorbic acid (AA) using silver nanoparticles as emission reagent was investigated. In this study, silver nanoparticles were prepared based on aqueous–gaseous phase reaction of silver nitrate solution and ammonia gas. The nanoparticles were water‐soluble, stable and had a narrow emission band. They were used as a fluorescence probe for the assay of ascorbic acid on its quenching effect on the emission of silver nanoparticles. The principal reason for quenching is likely to be a complexation between ascorbic acid and silver nanoparticles. The quenching mechanism was established by Stern–Volmer law. Under the optimum conditions, the quenched fluorescence intensity was linear with the concentration of ascorbic acid in the range of 4.1 × 10^?6 to 1.0 ×10^?4 m (r = 0.9985) with a detection limit of 1.0 × 10^?7 m . The RSD for repeatability of the sensor for the assay of ascorbic acid concentration of 3.0 × 10^?5 and 4.0 × 10^?6 m was found to be 1.5 and 1.3%, respectively. The proposed method was applied to the determination of ascorbic acid in vegetables and vitamin C tablets. Copyright © 2009 John Wiley & Sons, Ltd.     

A single-channel sensor based on gramicidin controlled by molecular recognition at bilayer lipid membranes containing receptor

A novel ion-channel sensor based on a membrane bound receptor and a single gramicidin channel is described, in which the binding of an analyte to the membrane bound receptor modulates the single-channel activity of gramicidin. The sensor is composed of a planar bilayer lipid membrane (BLM) containing biotin-labeled phosphatidylethanolamine as receptor for avidin and gramicidin as signal transducer. When the receptor catches an analyte (avidin or ferritin-labeled avidin (FA)) at the membrane surface, the bilayer structure is locally distorted and the gramicidin monomer/dimer kinetics is modulated in a manner that the fraction of channel opening with a short lifetime ( < or = 100 ms) to the total opening events increases. The fraction was found to increase with the concentration of avidin from 1.0 x 10(-9) to 1.0 x 10(-6) M and of FA from 1.0 x 10(-9) to 1.0 x 10(-8) M. With dinitrophenyl-labeled PE embedded as receptor in the BLM for monoclonal anti-dinitrophenyl antibody (anti-DNP), the fraction of channel openings ( < or = 100 ms) increased with the concentration of anti-DNP from 2.0 x 10(-9) to 2.0 x 10(-7) g/ml. Bovine serum albumin (BSA) and anti-BSA antibody caused no changes in the channel opening. The possible mechanism of analyte-induced modulation of single-channel activity of gramicidin is also discussed.     

Designing of MIP based QCM sensor having thymine recognition sites based on biomimicking DNA approach

Quartz crystal microbalance (QCM) sensors coated with molecular imprinted polymers (MIP) have been developed for the determination of thymine. In this method, methacryloylamidoadenine (MA-Ade) have used as a new monomer and thymine template for inspiration of DNA nucleobases interaction. The thymine can be simultaneously hydrogen binding to MA-Ade and fit into the shape-selective cavities. Thus, the interaction between nucleobases has an effect on the binding ability of the QCM sensors. The binding affinity of the thymine imprinted sensors has investigated by using the Langmuir isotherm. The thymine imprinted QCM electrodes have shown homogeneous binding sites for thymine (K_a: 1.0 × 10⁵ M⁻¹) while heterogeneous binding sites for uracil. On the other hand, recognition selectivity of the QCM sensor based on thymine imprinted polymer toward to uracil, ssDNA and ssRNA has been reported in this work.     

A portable gas sensor based on cataluminescence

We describe a portable gas sensor based on cataluminescence. Miniaturization of the gas sensor was achieved by using a miniature photomultiplier tube, a miniature gas pump and a simple light seal. The signal to noise ratio (SNR) was considered as the evaluation criteria for the design and testing of the sensor. The main source of noise was from thermal background. Optimal working temperature and flow rate were determined experimentally from the viewpoint of improvement in SNR. A series of parameters related to analytical performance was estimated. The limitation of detection of the sensor was 7 ppm (SNR = 3) for ethanol and 10 ppm (SNR = 3) for hydrogen sulphide. Zirconia and barium carbonate were respectively selected as nano‐sized catalysts for ethanol and hydrogen sulphide. Copyright © 2012 John Wiley & Sons, Ltd.     

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2+

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺. Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇/F₅₉₀ and the concentration of Hg²⁺. The detection range was 10–70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.     

A Schiff‐based sensor with turn‐on fluorescence for selective detection of Hg2+

A simple Schiff‐base colorimetric receptor 1 was prepared. It exhibits an ‘off–on‐type’ mode with high sensitivity in the presence of Hg²⁺. The change in color is very easily observed by the naked eye in the presence of Hg²⁺, whereas other metal cations do not induce such a change. A Job plot indicated a 1 : 1 complexation stoichiometry between receptor 1 and Hg²⁺. The association constant for 1–Hg²⁺ in Tetrahydrofuran (THF) was determined to be 1.3 × 10⁹ M^‐1 using a Hill plot. Copyright © 2013 John Wiley & Sons, Ltd.     

Electrochemical sensor for dopamine based on a novel graphene-molecular imprinted polymers composite recognition element

A novel composite of graphene sheets/Congo red-molecular imprinted polymers (GSCR-MIPs) was synthesized through free radical polymerization (FRP) and applied as a molecular recognition element to construct dopamine (DA) electrochemical sensor. The template molecules (DA) were firstly absorbed at the GSCR surface due to their excellent affinity, and subsequently, selective copolymerization of methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) was further achieved at the GSCR surface. Potential scanning was presented to extract DA molecules from the imprinted polymers film, and as a result, DA could be rapidly and completely removed by this way. With regard to the traditional MIPs, the GSCR-MIPs not only possessed a faster desorption and adsorption dynamics, but also exhibited a higher selectivity and binding capacity toward DA molecule. As a consequence, an electrochemical sensor for highly sensitive and selective detection of DA was successfully constructed as demonstration based on the synthesized GSCR-MIPs nanocomposites. Under experimental conditions, selective detection of DA in a linear concentration range of 1.0 × 10(-7)-8.3 × 10(-4)M was obtained, which revealed a lower limit of detection and wider linear response compared to some previously reported DA electrochemical MIPs sensors. The new DA electrochemical sensor based on GSCR-MIPs composites also exhibited excellent repeatability, which expressed as relative standard deviation (RSD) was about 2.50% for 30 repeated analyses of 20 μM DA.     

Characterization and application of PBA fiber optic chemical film sensor based on fluorescence multiple quenching

The three types of structure of the pyrenebutyric acid of fiber optic chemical film sensor were stud-ied by fluorescence multiple quenching. They are, for different test samples and purposes, respectively general, three-way and combined. A tri-cup method was designed to demonstrate the multiple quenching of response mechanism, and a relationship formula of mathematical approach was established. The response mechanism was shown to include the dynamic quenching , inner-filter effects and/or resonance energy transfer. To show the response characterization in a series of organic and inorganic quenchers, a new concept of apparent quenching coefficient Kq was advanced. This kind of sensor has been used in continuous and in situ monitoring of the dissolution rate of drug tablets, on line and in situ monitoring of some organic therapeutic drugs in biological fluid and Cr( VI ) in industrial waste water. The measured data were examined and compared with HPLC or HPTLCS. Test results show that the sensors and appa     

A highly selective and sensitive boronic acid-based sensor for detecting Pd2+ ion under mild conditions

Herein, a boronic acid-based sensor was reported selectively to recognize Pd²⁺ ion. The fluorescence intensity increased 36-fold after sensor binding with 2.47 × 10⁻⁵ M of Pd²⁺ ion. It was carried out in the 99% aqueous solution for binding tests, indicating sensor having good water solubility. In addition, it is discernible that Pd²⁺ ion turned on the blue fluorescence of sensor under a UV–lamp (365 nm), while other ions (Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cr²⁺, Cd²⁺, Co²⁺, Cs²⁺, Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Li⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺ and Zn²⁺) did not show the similar change. Furthermore, sensor has a low limit of detection (38 nM) and high selectivity, which exhibits the potential for the development of Pd²⁺ recognition in practical environments.     

A novel BOD sensor based on bacterial luminescence

A reagent-type BOD sensor with a new principle employing a luminous bacterium, Photobacterium phosphoreum, was developed. The increased intensity of luminescence resulting from the cellular assimilation of organic compounds in wastewater was detected by a photodiode. The BOD response of the bacterial reagent could be obtained within 15 min with +/-7% error. The temperature condition for optimal BOD response was 18 degrees to 25 degrees C at pH 7 to 8, indicating that it is possible to measure BOD at room temperature without having to stabilize the temperature of the measuring system. For practical use, two procedures for long-term preservation of the bacterial reagent, vacuum drying method and freezing method, are suggested. The metabolic characteristics of employed luminous bacteria were investigated by comparing the BOD values for several pure organic substrates estimated by the BOD sensor with conventional 5-day BOD values. In comparison with the 5-day measurement for some wastewater samples, BOD values estimated by the sensor showed comparatively good agreement with those measured by the 5-day method. (c) 1993 Wiley & Sons, Inc.     

A versatile colony assay based on NADH fluorescence

Direct visualization of the activity of enzymes expressed by bacterial colonies attached to a solid support, often referred to as “filter assay”, is a powerful strategy for the identification of new or improved biocatalysts. In this work we demonstrate the usefulness of NAD⁺/NADH coupled enzymatic reactions as visualization tool in such experimental setups. Dehydrogenases, capable of oxidizing or reducing the reaction product released from the bacterial colony were supplemented to the screening solution, together with the screening substrate and a sufficient amount of NAD⁺ or NADH, respectively. We also examined the screening of directly NAD⁺/NADH coupled reactions. The release or consumption of NADH in the area of colonies was monitored on behalf of its fluorescence at 450 nm. Excitation was achieved by standard “black-light” UV tubes (340–360 nm). The visible fluorescence signal was recorded using a CCD-camera. We got excellent results for the screening of threonine aldolases and esterases and were able to show the principle utility for amidase, nitrilase, nitrile hydratase, hydroxynitrile lyase and benzaldehyde dehydrogenase active colonies.     

A graphene oxide-peptide fluorescence sensor for proteolytically active prostate-specific antigen

This paper presents a novel sensor to detect proteolytically active prostate-specific antigen (PSA) by assembling a purpose-designed FITC-labeled peptide with graphene oxide (GO). The fluorescence of the dye-labeled peptide was quenched in the presence of GO. Reaction of the sensor with PSA cleaves the peptide, leading to the release of the dye moiety and a great increase in fluorescence intensity in a dose- and time-dependent manner, and PSA can be quantified accordingly. This approach is simple compared to existing methods since the GO-peptide-based sensor is easily assembled and detection can be achieved without the involvement of complicated procedures. Moreover, the applicability of the method has been demonstrated by detecting PSA in spiked urine samples.     

A fluorescence resonance energy transfer activation sensor for Arf6

The involvement of the small GTPase Arf6 in Rac activation, cell migration, and cancer invasiveness suggests that it is activated in a spatially and temporally regulated manner. Small GTPase activation has been imaged in cells using probes in which the GTPase and a fragment of a downstream effector protein are fused to fluorescent reporter proteins that constitute a fluorescence resonance energy transfer (FRET) donor/acceptor pair. Unlike other Ras family GTPases, the N terminus of Arf6 is critical for membrane targeting and, thus, cannot be modified by fusion to a fluorescent protein. We found that the previously described C-terminal green fluorescent protein (GFP) derivative also shows diminished membrane targeting. Therefore, we inserted a fluorescent protein into an inert loop within the Arf6 sequence. This fusion showed normal membrane targeting, nucleotide-dependent interaction with the downstream effector GGA3, and normal regulation by a GTPase-activating protein (GAP) and a guanine nucleotide exchange factor (GEF). Using the recently developed CyPET/YPET fluorescent proteins as a FRET pair, we found that Arf6-CyPET underwent efficient energy transfer when bound to YPET-GGA3 effector domain in intact cells. The addition of platelet-derived growth factor (PDGF) to fibroblasts triggered a rapid and transient increase in FRET, indicative of Arf6 activation. These reagents should be useful for investigations of Arf6 activation and function.