Paper-based sensor depending on the Prussian blue pH sensitivity: Smartphone-assisted detection of urea

DIW 3D printing of well-interconnected hierarchically porous microlattice for ultrasensitive photoelectrochemical sensing

Endowing channel-interconnected photoelectrode with prominent light-absorbing and analyte-trapping is pivotal but challenging for implementing high-performance photoelectrochemical sensing system. Herein, we demonstrated an effective and controllable direct ink writing (DIW) 3D printing coupled with molecular imprinting technology for consistently fabricating microlattice-shaped photoelectrochemical sensor, with multiscale well-interconnected channels created by accessible alliance of regular macrochannels originated from layer-by-layer assembly of printed filaments and abundant microchannels built by jointing molecule-recognized polyaniline (PANI), photoactive TiO2 and conductive graphene (G) nanosheets within filaments. The unique structure merit facilitated ready spreading of incident light into the sensor interior, and meanwhile enabled rapid diffusion/infiltration of analytes onto all specificity binding sites situated inside the sensor, thereby allowing light absorbance and analyte adsorption at a high level. As a result, the 3D-printed molecularly imprinted photoelectrochemical sensor displayed impressive monitoring capability for urea, with rapid response, low detection limit (2 μM), wide linear range (10-700 μM), exceptional selectivity and working stability. This work opens up a promising route for architecting advanced photoelectrochemical devices to cater to highly sensitive and selective sensing.

4D-printed shape-programmable [H+]-responsive needles for determination of urea

Four-dimensional printing (4DP) technologies are revolutionizing the fabrication, functionality, and applicability of stimuli-responsive analytical devices. More practically, 4DP technologies are effective in fabricating devices with complex geometric designs and functions, and the degree of shape programming of 4D-printed stimuli-responsive devices can be optimized to become a reliable analytical strategy. Although shape-programming modes play a critical role in determining the analytical characteristics of 4D-printed stimuli-responsive sensing devices, the effect of shape-programming modes on the analytical performance of 4D-printed stimuli-responsive devices remains an unexplored subject. We employed digital light processing three-dimensional printing (3DP) with acrylate-based photocurable resins and 2-carboxyethyl acrylate (CEA)-incorporated photocurable resins for 4DP of the bending, helixing, and twisting needles. Upon immersion in samples with pH values above the pKa of CEA, the electrostatic repulsion among the dissociated carboxyl groups of polyCEA caused swelling of the CEA-incorporated part and [H+]-dependent shape programming. When coupling with the derivatization reaction of the urease-mediated hydrolysis of urea, the decline in [H+] induced shape programming of the needles, offering reliable determination of urea based on the shape-programming angles. After optimizing the experimental conditions, the helixing needles provided the best analytical performance, with the method's detection limit of 0.9 μM. The reliability of this analytical method was validated by determining urea in samples of human urine and sweat, fetal bovine serum, and rat plasma with spike analyses and comparing these results with those obtained from a commercial assay kit. Our demonstration and analytical results suggest the importance of optimizing the shape-programming modes to improve the analytical performance of 4D-printed stimuli-responsive shape-programming sensing devices and emphasize the benefits and applicability of 4DP technologies in advancing the development and fabrication of stimuli-responsive sensing devices for chemical sensing and quantitative chemical analyses.

Progress in the design of portable colorimetric chemical sensing devices

The need for precise determination of heavy metals, anions, biomolecules, pesticides, drugs, and other substances is vital across clinical, environmental, and food safety domains. Recent years have seen significant progress in portable colorimetric chemical sensing devices, revolutionizing on-the-spot analysis. This review offers a comprehensive overview of these advancements, covering handheld colorimetry, RGB-based colorimetry, paper-based colorimetry, and wearable colorimetry devices. It explores the underlying principles, functional materials (chromophoric reagents/dyes and nanoparticles), detection mechanisms, and their applications in environmental monitoring, clinical care, and food safety. Noble metal nanoparticles (NPs) have arisen as promising substitutes in the realm of sensing materials. They display notable advantages, including heightened sensitivity, the ability to fine-tune their plasmonic characteristics for improved selectivity, and the capacity to induce visible color changes, and simplifying detection. Integration of NPs fabricated paper device with smartphones and wearables facilitates reagent-free, cost-effective, and portable colorimetric sensing, enabling real-time analysis and remote monitoring.

A facile and sensitive magnetic relaxation sensing strategy based on the conversion of Fe3+ ions to Prussian blue precipitates for the detection of alkaline phosphatase and ascorbic acid oxidase

Herein, a novel magnetic relaxation sensing strategy based on the change in Fe³⁺ content has been proposed by utilizing the conversion of Fe³⁺ ions to Prussian blue (PB) precipitates. Compared with the common detection approach based on the valence state change of Fe³⁺ ions, our strategy can cause a larger change in the relaxation time of water protons and higher detection sensitivity since PB precipitate can induce a larger change in the Fe³⁺ ion concentration and has a weaker effect on the relaxation process of water protons relative to Fe²⁺ ions. Then, we employ alkaline phosphatase (ALP) as a model target to verify the feasibility and detection performance of the as-proposed strategy. Actually, ascorbic acid (AA) generated from the ALP-catalyzed L-ascorbyl-2-phosphate hydrolysis reaction can reduce potassium ferricyanide into potassium ferrocyanide, and potassium ferrocyanide reacts with Fe³⁺ to form PB precipitates, leading to a higher relaxation time. Under optimum conditions, the method for ALP detection has a wide linear range from 5 to 230 mU/mL, and the detection limit is 0.28 mU/mL, sufficiently demonstrating the feasibility and satisfactory analysis performance of this strategy, which opens up a new path for the construction of magnetic relaxation sensors. Furthermore, this strategy has also been successfully applied to ascorbic acid oxidase detection, suggesting its expansibility in magnetic relaxation detection.

A Paper-Based Analytical Device Integrated with Smartphone: Fluorescent and Colorimetric Dual-Mode Detection of β-Glucosidase Activity

In this work, indoxyl-glucoside was used as the substrate to develop a cost-effective, paper-based analytical device for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity through a smartphone. The β-glucosidase can hydrolyze the colorless substrate indoxyl-glucoside to release indoxyl, which will be self-oxidized to generate green products in the presence of oxygen. Meanwhile, the green products emit bright blue-green fluorescence under ultraviolet-visible light irradiation at 365 nm. Fluorescent or colorimetric images were obtained by a smartphone, and the red-green-blue channels were analyzed by the Adobe Photoshop to quantify the β-glucosidase activity. Under the optimum conditions, the relative fluorescent and colorimetric signals have a good linear relationship with the activity of β-glucosidase, in the range of 0.01-1.00 U/mL and 0.25-5.00 U/mL, and the limits of detection are 0.005 U/mL and 0.0668 U/mL, respectively. The activities of β-glucosidase in a crude almond sample measured by the fluorescent and colorimetric methods were 23.62 ± 0.53 U/mL and 23.86 ± 0.25 U/mL, respectively. In addition, the spiked recoveries of normal human serum and crude almond samples were between 87.5% and 118.0%. In short, the paper-based device, combined with a smartphone, can provide a simple, environmentally friendly, and low-cost method for the fluorescent and colorimetric dual-mode detection of β-glucosidase activity.