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A facile colorimetric method for rapid detection of putrescine (Put) and cadaverine (Cad) based on the peroxidase-like activity of (Fe,Co) codoped-CDs was developed. With the enzymatic hydrolysis by diamine oxidase (EC 1.4.3.6, DAO), Biogenic amines (BAs) were decomposed to produce H2O2 which oxidizes 3,3',5,5'-tetramethyl-benzidine with the catalysis of (Fe,Co)-codoped CDs. By means of principal component analysis, Put and Cad were differentiated from BAs and their total amounts were detected by measuring the spectral absorption at 652 nm with a linear range from 0.25 to 10 mg kg−1. The colorimetric method was applied to the detection of Put and Cad in various fish samples with a detection limit ((3σ/k, n = 9)) of 0.06 mg kg−1. Besides verifying with HPLC, the recoveries of the colorimetric method were also tested by spiking standards at three levels with the range of 98.2%–115.7%, indicating that the established colorimetric method was accurate and sensitive.

A facile and practical ratiometric fluorescence probe based on two CdTe quantum dots (QDs) coated with molecularly imprinted polymers (MIPs) was prepared for the detection of trace malachite green (MG) in fish. Two CdTe QDs coated with MIPs were fabricated by a one-pot method using MG, (3-aminopropyl) triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) as template, functional monomer, and cross-linker, respectively. CdTe QDs with λem 530 nm (gQDs) and 630 nm (rQDs) were used as the referential fluorophore and target sensitive fluorophore, respectively. The fluorescence intensity of gQDs remained unchanged in the presence of MG, while the fluorescence of rQDs could be quantitatively quenched by MG based on the strategy of fluorescence resonance energy transfer. The ratiometric fluorescence probe (MIPs@gQDs&rQDs) was characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The linear range of MG detection was 0.1–32 μmol L−1 with a detection limit of 8.8 μg kg−1. The constructed probe has been successfully applied to the detection of MG in fish with the recoveries of 92.3–109.1%, which were validated by the method of HPLC. The result indicated that the probe possessed rapid response, wide linear range, high sensitivity, and relatively high selectivity, and was low-cost and easy in operation in the detection of MG in fish samples.

Electrochemiluminescence (ECL) integrates the advantages of electrochemical detection and chemiluminescent techniques. The method has received particular attention because it is highly sensitive and selective, has a wide linear range but low reagent costs. The use of nanomaterials with their unique physical and chemical properties has led to new kinds of biosensors that exhibit high sensitivity and stability. Compared to other nanomaterials, DNA nanostructures are more biocompatible, more hydrophilic, and thus less prone to nonspecific adsorption onto the electrode surface. We describe here a label-free and ultrasensitive ECL biosensor for detecting a cancer-associated microRNA at a femtomolar level. We have designed two auxiliary probes that cause the formation of a long-range self-assembly in the form of a μm-long 1-dimensional DNA concatamer. These can be used as carriers for signal amplification. The intercalation of the ECL probe Ru(phen)3 2+ into the grooves of the concatamers leads to a substantial increase in ECL intensity. This amplified sensor shows high selectivity for discriminating complementary target and other mismatched RNAs. The biosensor enables the quantification of the expression of microRNA-21 in MCF-7 cells. It also displays very low limits of detection and provides an alternative approach for the detection of RNA or DNA detection in diagnostics and gene analysis.