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Fundamental understanding of microRNA (miRNA) regulated cellular processes requires smart tools capable of imaging the expression and distribution of miRNAs in living cells. However, the low-expression of miRNA and a complex intracellular environment bring great challenges for intracellular miRNA imaging. We found that boron doped g-C3N4 nanosheets (BCNNS) are capable of binding copper nanocluster (CuNC)-labeled hairpin DNA and quench the fluorescence of CuNCs. A turn-on fluorescent sensor for the detection of miR-582-3p in living cells is thus developed based on the affinity change of BCNNS and DNA probes upon their recognition to the target. Ratiometric fluorescence measurement is achieved by taking advantage of the inherent fluorescence emission of BCNNS as the cell interior label. By combining the hybridization chain reaction (HCR) amplification and inherent sensitivity of a turn-on fluorescence sensing strategy, this approach gives rise to a limit of detection (LOD) of 49 fmol L-1 for miR-582-3p, within a linear range of 0.2-1 pmol L-1. The LOD is further improved to 12 fmol L-1 with a correlation to ICP-MS by measuring the 63Cu isotope. With the merits of high sensitivity and selectivity of the HCR and the anti-interfering capability of the ratiometric fluorescent probe, the single cell miR-582-3p expression levels are successfully detected, which indicated that this assay provides a sensitive and enzyme-free approach for miRNA imaging in living cells.

In recent years, α-glucosidase inhibitors (AGIs) have played a significant role in the treatment of type II diabetes (T2D), so it is necessary to develop a reliable and sensitive method to find new AGIs. Herein, we establish a novel method based on fluorescent carbon nitride nanoparticles (CNNPs) for the sensitive detection of the activity of α-glucosidase (α-glu) and the screening of its inhibitors. A CNNP-based fluorescent probe is synthesized from green raw materials, urea and lysine, by a one-pot method. In the presence of α-glu, the substrate 4-nitrophenyl-α-D-glucopyranoside (pNPG) is hydrolyzed to generate 4-nitrophenol (pNP), leading to the fluorescence (FL) quenching of CNNPs due to the inner filter effect (IFE). On the other hand, the activity of α-glu is inhibited after the addition of AGIs, which turns on the FL of CNNPs. In this way, the detection of α-glu activity and the screening of AGIs are achieved. The linear range is 1.25–10.00 U L−1 with a limit of detection as low as 0.17 U L−1 and the IC50 values of two typical inhibitors (gallic acid and acarbose) are 813 μM and 465 μM, respectively. The CNNP probe is further applied for the determination of α-glu activity in human serum samples with satisfactory results.

In this work, ZnO nanorods (ZnO NRs) with different sizes were hydrothermally grown on the surface of Whatman filter paper for the fabrication of a microfluidic paper-based device (μPAD) for the simultaneous detection of glucose and uric acid. As dual enzymatic reaction was employed for the colorimetric detection in this μPAD, the presence of ZnO NRs promoted the enzyme immobilization thus significantly enhancing the colorimetric signal. The coffee ring effect was effectively conquered by the uniform distribution of ZnO NR as well as a specialized double-layered μPAD design. Meanwhile, two color indicators with distinct colors were used to provide complementary results to better quantify the concentration of the analytes by naked eye. As a result, two linear calibration curves were obtained for the detection of glucose (0.01–10 mmol L−1) and uric acid (0.01–5 mmol L−1), along with a LOD of 3 μmol L−1 for glucose and 4 μmol L−1 for uric acid, respectively. The practical usefulness of the proposed μPAD was further validated by the simultaneous analysis of glucose and uric acid in serum samples and urine samples.

Density functional calculations have been performed to investigate the acidities for the carboxylated single-wall carbon nanotubes and graphene. The pKa values for different COOH-functionalized models with varying lengths, diameters and chirality of nanotubes and with different edges of graphene were predicted using the SMD/M05-2X/6-31G* method combined with two universal thermodynamic cycles. The effects of following factors, such as, the functionalized position of carboxyl group, the Stone-Wales and single vacancy defects, on the acidity of the functionalized nanotube and graphene have also been evaluated. The deprotonated species have undergone decarboxylation when the hybridization mode of the carbon atom at the functionalization site changed from sp2 to sp3 both for the tube and graphene. The knowledge of the pKa values of the carboxylated nanotube and graphene could be of great help for the understanding of the nanocarbon materials in many diverse areas, including environmental protection, catalysis, electrochemistry and biochemistry.

Nanozymes have emerged as promising alternatives to overcome the high cost and low stability issues of natural enzymes. Particularly, those with peroxidase-like activities have been extensively studied to construct versatile biosensors. In this article, we demonstrate that the modification of the graphitic carbon nitride nanosheets (g-C3N4 nanosheets) by plasmonic gold nanoparticles (AuNPs) greatly enhances its catalytic performance as peroxidase mimetic. In the presence of H2O2, the AuNPs@g-C3N4 nanosheets can catalyze the redox reaction of 3,3′,5,5′- tetramethylbenzidine (TMB) to produce a blue color. Based on the observation, a colorimetric sensing method for glucose is further developed with the assistance of glucose oxidase (GOx). The linear range for glucose is from 5 to 100 μmol L−1 (R2 = 0.9967) and the limit of detection (LOD) is 1.2 μmol L−1. The LOD can be further lowered down to 0.75 μmol L−1 by using H2SO4 as termination agent and measuring the absorbance of the yellow product at λ = 451 nm. Moreover, the practical usefulness of AuNPs@g-C3N4 nanosheets as a peroxidase nanozyme for glucose determination in human serum and urine is also demonstrated.