Sensitive detection of biothiols and histidine based on the recovered fluorescence of the carbon quantum dots–Hg(II) system

In this paper, we presented a novel, rapid and highly sensitive sensor for glutathione (GSH), cysteine (Cys) and histidine (His) based on the recovered fluorescence of the carbon quantum dots (CQDs)–Hg(II) system. The CQDs were synthesized by microwave-assisted approach in one pot according to our previous report. The fluorescence of CQDs could be quenched in the presence of Hg(II) due to the coordination occurring between Hg(II) and functional groups on the surface of CQDs. Subsequently, the fluorescence of the CQDs–Hg(II) system was recovered gradually with the addition of GSH, Cys or His due to their stronger affinity with Hg(II). A good linear relationship was obtained from 0.10 to 20 μmol L⁻¹ for GSH, from 0.20 to 45 μmol L⁻¹ for Cys and from 0.50 to 60 μmol L⁻¹ for His, respectively. This method has been successfully applied to the trace detection of GSH, Cys or His in human serum samples with satisfactory results. The proposed method was simple in design and fast in operation, which demonstrated great potential in bio-sensing fields.     

Facile synthesis of nitrogen-doped carbon dots for Fe sensing and cellular imaging

A fast and facile approach to synthesize highly nitrogen (N)-doped carbon dots (N-CDs) by microwave-assisted pyrolysis of chitosan, acetic acid and 1,2-ethylenediamine as the carbon source, condensation agent and N-dopant, respectively, is reported. The obtained N-CDs are fully characterized by elemental analysis, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction pattern, X-ray photoelectron spectroscopy, UV–vis absorption, and photoluminescence spectroscopy. Doping N heteroatoms benefits the generation of N-CDs with stronger fluorescence emission. As the emission of N-CDs is efficiently quenched by Fe³⁺, the as-prepared N-CDs are employed as a highly sensitive and selective probe for Fe³⁺ detection. The detection limit can reach as low as 10 ppb, and the linear range is 0.010–1.8 ppm Fe³⁺. The as-synthesized N-CDs have been successfully applied for cell imaging and detecting Fe³⁺ in biosystem.     

Highly selective and sensitive paper-based colorimetric sensor using thiosulfate catalytic etching of silver nanoplates for trace determination of copper ions

A novel, highly selective and sensitive paper-based colorimetric sensor for trace determination of copper (Cu²⁺) ions was developed. The measurement is based on the catalytic etching of silver nanoplates (AgNPls) by thiosulfate (S₂O₃²⁻). Upon the addition of Cu²⁺ to the ammonium buffer at pH 11, the absorption peak intensity of AuNPls/S₂O₃²⁻ at 522 nm decreased and the pinkish violet AuNPls became clear in color as visible to the naked eye. This assay provides highly sensitive and selective detection of Cu²⁺ over other metal ions (K⁺, Cr³⁺, Cd²⁺, Zn²⁺, As³⁺, Mn²⁺, Co²⁺, Pb²⁺, Al³⁺, Ni²⁺, Fe³⁺, Mg²⁺, Hg²⁺ and Bi³⁺). A paper-based colorimetric sensor was then developed for the simple and rapid determination of Cu²⁺ using the catalytic etching of AgNPls. Under optimized conditions, the modified AgNPls coated at the test zone of the devices immediately changes in color in the presence of Cu²⁺. The limit of detection (LOD) was found to be 1.0 ng mL⁻¹ by visual detection. For semi-quantitative measurement with image processing, the method detected Cu²⁺ in the range of 0.5–200 ng mL⁻¹(R² = 0.9974) with an LOD of 0.3 ng mL⁻¹. The proposed method was successfully applied to detect Cu²⁺ in the wide range of real samples including water, food, and blood. The results were in good agreement according to a paired t-test with results from inductively coupled plasma-optical emission spectrometry (ICP-OES).     

Optofluidic microsystem with quasi-3 dimensional gold plasmonic nanostructure arrays for online sensitive and reproducible SERS detection

Practical applications of chemical and biological detections through surface-enhanced Raman scattering (SERS) require high reproducibility, sensitivity, and efficiency, along with low-cost, straightforward fabrication. In this work, we integrated a poly-(dimethylsiloxane) (PDMS) chip with quasi-3D gold plasmonic nanostructure arrays (Q3D-PNAs), which serve as SERS-active substrates, into an optofluidic microsystem for online sensitive and reproducible SERS detections. The Q3D-PNA PDMS chip was fabricated through soft lithography to ensure both precision and low-cost fabrication. The optimal dimension of the Q3D-PNA in PDMS was designed using finite-difference time-domain (FDTD) electromagnetic simulations with a simulated enhancement factor (EF) of 1.6 × 10⁶. The real-time monitoring capability of the SERS-based optofluidic microsystem was investigated by kinetic on/off experiments through alternatively flowing Rhodamine 6G (R6G) and ethanol in the microfluidic channel. A switch-off time of ∼2 min at a flow rate of 0.3 mL min⁻¹ was demonstrated. When applied to the detection of low concentration malathion, the SERS-based optofluidic microsystem with Q3D-PNAs showed high reproducibility, significantly improved efficiency and higher detection sensitivity via increasing the flow rate. The optofluidic microsystem presented in this paper offers a simple and low-cost approach for online, label-free chemical and biological analysis and sensing with high sensitivity, reproducibility, efficiency, and molecular specificity.     

Ratiometric fluorescence transduction by hybridization after isothermal amplification for determination of zeptomole quantities of oligonucleotide biomarkers with a paper-based platform and camera-based detection

Paper is a promising platform for the development of decentralized diagnostic assays owing to the low cost and ease of use of paper-based analytical devices (PADs). It can be challenging to detect on PADs very low concentrations of nucleic acid biomarkers of lengths as used in clinical assays. Herein we report the use of thermophilic helicase-dependent amplification (tHDA) in combination with a paper-based platform for fluorescence detection of probe-target hybridization. Paper substrates were patterned using wax printing. The cellulosic fibers were chemically derivatized with imidazole groups for the assembly of the transduction interface that consisted of immobilized quantum dot (QD)–probe oligonucleotide conjugates. Green-emitting QDs (gQDs) served as donors with Cy3 as the acceptor dye in a fluorescence resonance energy transfer (FRET)-based transduction method. After probe-target hybridization, a further hybridization event with a reporter sequence brought the Cy3 acceptor dye in close proximity to the surface of immobilized gQDs, triggering a FRET sensitized emission that served as an analytical signal. Ratiometric detection was evaluated using both an epifluorescence microscope and a low-cost iPad camera as detectors. Addition of the tHDA method for target amplification to produce sequences of ∼100 base length allowed for the detection of zmol quantities of nucleic acid targets using the two detection platforms. The ratiometric QD-FRET transduction method not only offered improved assay precision, but also lowered the limit of detection of the assay when compared with the non-ratiometric QD-FRET transduction method. The selectivity of the hybridization assays was demonstrated by the detection of single nucleotide polymorphism.     

Tutorial review on validation of liquid chromatography–mass spectrometry methods: Part I

This is the part I of a tutorial review intending to give an overview of the state of the art of method validation in liquid chromatography mass spectrometry (LC–MS) and discuss specific issues that arise with MS (and MS/MS) detection in LC (as opposed to the “conventional” detectors). The Part I briefly introduces the principles of operation of LC–MS (emphasizing the aspects important from the validation point of view, in particular the ionization process and ionization suppression/enhancement); reviews the main validation guideline documents and discusses in detail the following performance parameters: selectivity/specificity/identity, ruggedness/robustness, limit of detection, limit of quantification, decision limit and detection capability. With every method performance characteristic its essence and terminology are addressed, the current status of treating it is reviewed and recommendations are given, how to determine it, specifically in the case of LC–MS methods.     

Electrochemical droplet-based microfluidics using chip-based carbon paste electrodes for high-throughput analysis in pharmaceutical applications

This paper presents the first example of a pharmaceutical application of droplet-based microfluidics coupled with chronoamperometric detection using chip-based carbon paste electrodes (CPEs) for determination of dopamine (DA) and ascorbic acid (AA). Droplets were generated using an oil flow rate of 1.80 μL min⁻¹, whereas a flow rate of 0.80 μL min⁻¹ was applied to the aqueous phase, which resulted in a water fraction of 0.31. The optimum applied potential for chronoamperometric measurements in droplets was found to be 150 mV. Highly reproducible analysis of DA and AA was achieved with relative standard deviations of less than 5% for both intra-day and inter-day measurements. The limit of detection (LOD) and limit of quantitation (LOQ) were found to be 20 and 70 μM for DA and 41 and 137 μM for AA, respectively. Linearity of this method was in the ranges of 0.02–3.0 mM for DA and 0.04–3.0 mM for AA. This system was successfully applied to determine the amounts of DA and AA in intravenous drugs. Calibration curves of DA and AA for quantitative analysis were obtained with good linearity with R² values of 0.9984 and 0.9988, respectively. Compared with the labeled amounts, the measured concentrations of DA and AA obtained from this system were insignificantly different, with error percentages of less than ±3.0%, indicating a high accuracy of the developed method.     

Amperometric l-glutamate biosensor based on bacterial cell-surface displayed glutamate dehydrogenase

A novel l-glutamate biosensor was fabricated using bacteria surface-displayed glutamate dehydrogenase (Gldh-bacteria). Here the cofactor NADP⁺-specific dependent Gldh was expressed on the surface of Escherichia coli using N-terminal region of ice nucleation protein (INP) as the anchoring motif. The cell fractionation assay and SDS-PAGE analysis indicated that the majority of INP-Gldh fusion proteins were located on the surface of cells. The biosensor was fabricated by successively casting polyethyleneimine (PEI)-dispersed multi-walled carbon nanotubes (MWNTs), Gldh-bacteria and Nafion onto the glassy carbon electrode (Nafion/Gldh-bacteria/PEI-MWNTs/GCE). The MWNTs could not only significantly lower the oxidation overpotential towards NAPDH, which was the product of NADP⁺ involving in the oxidation of glutamate by Gldh, but also enhanced the current response. Under the optimized experimental conditions, the current–time curve of the Nafion/Gldh-bacteria/PEI-MWNTs/GCE was performed at +0.52 V (vs. SCE) by amperometry varying glutamate concentration. The current response was linear with glutamate concentration in two ranges (10 μM–1 mM and 2–10 mM). The low limit of detection was estimated to be 2 μM glutamate (S/N = 3). Moreover, the proposed biosensor is stable, specific, reproducible and simple, which can be applied to real samples detection.     

Novel gold nanoparticle trimer reporter probe combined with dry-reagent cotton thread immunoassay device for rapid human ferritin test

We reported here for the first time on the use of cotton thread combined with novel gold nanoparticle trimer reporter probe for low-cost, sensitive and rapid detection of a lung cancer related biomarker, human ferritin. A model system comprising ferritin as an analyte and a pair of monoclonal antibodies was used to demonstrate the proof-of-concept on the dry-reagent natural cotton thread immunoassay device. Results indicated that the using of novel gold nanoparticle trimer reporter probe greatly improved the sensitivity comparing with traditional gold nanoparticle reporter probe on the cotton thread immunoassay device. The assay avoids multiple incubation and washing steps performed in most conventional protein analyses. Although qualitative tests are realized by observing the color change of the test zone, quantitative data are obtained by recording the optical responses of the test zone with a commercial scanner and corresponding analysis software. Under optimal conditions, the cotton thread immunoassay device was capable of measuring 10 ng/mL human ferritin under room temperature which is sensitive enough for clinical diagnosis. Moreover, the sample solution employed in the assays is just 8 μL, which is much less than traditional lateral flow strip based biosensors.     

Reversed phase liquid chromatographic determination of organic acids using on-line complexation with copper(II) ion

We describe a simple and sensitive liquid chromatographic method for the analysis of organic acids using on-line complexation with copper(II) ion. Organic acids complexed with copper(II) ion were separated on a reversed-phase C18 column and detected by UV absorption at 240 nm. The copper(II) ion concentration in the mobile phase had a great influence on separation and sensitivity. A mobile phase consisting of 10 mM copper(II) sulfate in 5 mM sulfuric acid (pH 2.3) was used to separate nine organic acids (tartaric, malic, malonic, lactic, acetic, citric, maleic, succinic and fumaric acids). The detection limits of the examined organic acids calculated at S/N = 3 ranged from 0.6 to 100 μM. The detector signal was linear over three orders of magnitude of organic acid concentration. The method successfully measured organic acids in juice and vinegar samples.