Zhi-Yong Wu

Accurate identification of mercury species in aqueous environments is essential for assessing ecological and human health risks, given the distinct toxicity and bioaccumulation potential of different mercury forms. Herein, we present a paper-based analytical device that uses electrophoresis and surface-enhanced Raman scattering (SERS) for rapid and in situ detection of inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) in aqueous samples. Using a dumbbell-shaped paper microchannel and applying voltage to both ends of the channel, quickly separate Hg²⁺ and CH₃Hg⁺ due to their differential migration behaviors. The two mercury species were subsequently identified using AuNPs@β-cyclodextrin@4-mercaptophenylboronic acid (AuNPs@β-CD@4-MPBA) SERS tags, which enable distinct chemical recognition: electrophilic substitution by Hg²⁺ and Au-Hg amalgam formation with CH₃Hg⁺. The platform achieves sub μg L^-1 detection limits, completes the full workflow─from sample loading, mercury species separation, to spectra readout─in 5 min, and shows high specificity against common metal-ion interferents. Validation of mercury species in the seawater sample confirmed the robustness of the platform in complex matrices. This electrophoresis-assisted SERS platform provides a new strategy for online separation and identification of multiform toxic species.

The direct MS detection of amino acids obtained from serum was successfully demonstrated via a paper-based fast electrokinetic sample clean-up method.

Taking advantage of the intrinsic photocatalysis of TiO₂, hydrophilic reactor arrays were lithographically patterned on a hydrophobic paper via a simple UV irradiation. As a proof-of-concept, alkaline phosphatase (ALP) was used as the model analyte for colorimetric analysis. As ALP can induce hydrolysis of pyrophosphate-Zn(II) framework, the released Zn²⁺ ions are subsequently coordinated with red-colored zincon to form blue-colored zincon-Zn(II) chelate complex, and these color differences were applied for further colorimetric assay. The sensing platform showed response to ALP ranging from 20 ~ 800 U L⁻¹ with a detection limit of 3 U L⁻¹, and the recoveries of ALP in serum samples were in the range 95.7 ~ 104.5% with relative standard deviations from 2.10 to 3.84%. Additionally, the distinct wettability features of the proposed sensing platform effectively prevent lateral fluid spread out of hydrophilic reactors, thus allowing not only the use of minimum amount of analyte but it has also a high potential for simultaneous quantification of multiple samples. Graphical abstract

Charged species from complex media could be separated and enriched taking advantage of ion concentration polarization (ICP) effect; thus, ICP can be used for sample purification and improvement of detection sensitivity. In this paper, a novel and reliable ICP interface was established on a paper-based analytical device (PAD) by using ion exchange membrane, and electrokinetic stacking of target analytes from salty media was successfully demonstrated. Steady ICP effect was well observed in aqueous solution with up to 400 mM NaCl as shown by a fluorescent probe, which makes it possible to directly process salty physiological samples such as blood and urine with this type of PAD. Application of this method was demonstrated by direct online stacking of total protein from urine samples and image-based colorimetric detection by a smartphone camera. The linear response was in the range of 50–350 mg/L (R² = 0.99), with recovery rate in the range of 94.8–107.6% and relative standard deviation below 7.1%. The obtained results were consistent with that of the clinical method. As an off-line sample pretreatment method, the feasibility for rapid tandem mass spectrometry detection of amino acids from serum samples was also investigated, and promising results were obtained. This PAD method is of low cost, easy to operate, and reliable. As a disposable PAD, it is useful not only for sensitive point-of-care testing but also for direct purification and concentration of complex and highly conductive physiological samples for fast and accurate detection with advanced analytical instruments.