Analyte-triggered in situ "off-on" of Tyndall effect for smartphone-based quantitative nanosensing of Ag+ ions.

This work describes two new colorimetric methods for smartphone-based point-of-care nanosensing of toxic Ag+ ions. They were based on the analyte-triggered in situ "off-on" of Tyndall effect (TE) of non-plasmonic colloid or plasmonic metal nanoprobes. The first TE-inspired assay (TEA) focused on the initial analytical application of precipitation reactions where a non-plasmonic AgCl colloid could be formed once mixing the analyte with a NaCl solution. Such AgCl colloid displayed strong visual TE signals after their irradiation by a laser pointer pen, which unexpectedly achieved a detection limit of ~ 400 nM. The second TEA was further designed to reduce the limit down to ~ 78 nM using the analyte's oxidizability towards 3,3',5,5'-tetramethylbenzidine molecules. The redox reaction could create positively charged products that could make negatively charged plasmonic gold nanoparticles aggregate through electrostatic interactions to remarkably amplify their TE responses. Both limits were lower than the minimum allowable Ag+ level (~ 460 nM) in drinking water issued by the World Health Organization. The satisfactory recovery results for detecting Ag+ ions in river, pond, tap, and drinking water additionally demonstrated good selectivity, accuracy and practicality of the proposed methods for potential point-of-need uses in environmental analysis, public health, water safety, etc.

Chemiluminescent Imaging Assay of SARS-CoV-2 Protein with Target-Induced Enzyme Activity Regulation.

Simple but robust testing assays are essential for screening and diagnosis of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in COVID-19 pandemic. Here, we described a chemiluminescent imaging assay (CLIA) for sensitive and convenient detection of SARS-CoV-2 nucleocapsid protein (NP) by a target-induced enzyme activity regulation (T-EAR) strategy. The T-EAR used a pair of antibody-DNA probes to recognize SARS-CoV-2 NP and proximity-induce rolling circle amplification for mass-production of pyrophosphate to coordinate with Cu2+ , which prevented the reduction of Cu2+ to Cu+ by sodium ascorbate as well as the Cu+ -caused inactivation of horseradish peroxidase (HRP). The activity retention of HRP produced strong CL signal for the detection of SARS-CoV-2 NP by catalyzing the oxidation of luminol by H2 O2 . The T-EAR based CLIA showed a wide detection range from 1 pg/mL to 100 ng/mL (13 fM to 1.3 nM) with the requirement of only 0.75 µL of sample. This CLIA had advantages of good sensitivity, simple wash-free operation, acceptable accuracy, and high-throughput imaging detection, displaying potential applicability in screening assay of COVID-19 infection.

Highly Sensitive Colorimetric Detection of a Variety of Analytes via the Tyndall Effect.

This work initially reports the use of a quite familiar optical phenomenon of colloidal solutions, namely, the Tyndall Effect (TE) as signal readout for highly sensitive colorimetric chemical and biological analysis. Taking gold nanoparticles (GNPs) as a model colloid, the TE-inspired assay (TEA) is developed based on the conversion of a specific recognition event (e.g., the aptamer-analyte binding) into the aggregation of GNPs, leading to a significant TE enhancement. In the TEA, a cheap laser pointer pen is used as a hand-held light source, while a smartphone serves as a portable quantitative reader. The results show that the TE signaling strategy achieves a ∼1000-fold sensitivity improvement compared with the most common surface plasmon resonance signaling method using GNPs. The utility of the TEA is well demonstrated with the inexpensive, rapid, and portable detection of trace levels of analytes ranging from an important small-molecule drug (cocaine, ∼1.5 pM detection limit) to a protein biomarker (interferon-γ, ∼2.2 fM detection limit) and a toxic metal ion (Ag+, ∼1.4 nM detection limit). In addition, as the TE enhancement simply stems from the aggregation of either bare (unmodified) or modified GNPs, the TEA is universally applicable to almost all of the existing GNP-based liquid-phase colorimetric assays. The TEA method developed herein lights a new way for equipment-free point-of-care analysis in various fields including medical diagnosis, food safety evaluation, and environmental monitoring, especially in the resource-poor areas of the world.

Chemiluminescent screening of specific hybridoma cells via a proximity-rolling circle activated enzymatic switch.

The mass-production capability of hybridoma technology is bottlenecked by the routine screening procedure which is time-consuming and laborious as the requirement of clonal expansion. Here, we describe a 1-day chemiluminescent screening protocol for specific hybridoma cells on conventional 96-well plate via a proximity-rolling circle activated enzymatic switch (P-RCAES) strategy. The P-RCAES uses a pair of antigen-DNA probes to recognize secreted specific antibody and proximity-induce rolling circle amplification for mass-production of pyrophosphate to activate Cu(II) inhibited horseradish peroxidase and generate a strong chemiluminescent signal. The P-RCAES based homogeneous chemiluminescent assay can detect antibody down to 18 fM, and enables the screening of specific hybridoma cells secreting PCSK9 antibody at single-cell level without tedious cloning process. The proposed fast screening protocol has good expansibility without need of sophisticated instruments, and provides a screening method for greatly improving the efficiency of hybridoma technology.

Confined electrochemiluminescence imaging microarray for high-throughput biosensing of single cell-released dopamine.

The quantitative detection of single cell secretions is always limited by their accurate collection and the heterogeneity of different cells. In this work, a confined electrochemiluminescence (ECL) imaging microarray (CEIM) chip was designed to capture single or a few cells in each cylindrical microwell for high-throughput quantitation of cell-secreted dopamine (DA). The ITO surface at the bottom of microwells was functionalized with the film of DA aptamer modified coreactant-embedded polymer dots (Pdots), which endowed the chip with the abilities to both in situ recognize the target DA secreted from the cells and emit the ECL signal for responding the secreted target without need of any additional coreactant. At the applied potential of +1.4 V, the Pdots in the film emitted strong ECL signal, which could be quenched by the electrochemical oxidation product of DA in individual microwell for sensitive detection of single cell-released DA. The practicability of the proposed CEIM chip along with the ECL imaging and biosensing strategy was demonstrated by evaluating the amounts of single cell-released DA in different microwells under hypoxia stimulation. This protocol revealed the heterogeneity of cell secretion, and could be extended for quantitation of other secretions from different kinds of single cells.

Refillable Fuel-Loading Microshell Motors for Persistent Motion in a Fuel-Free Environment.

Artificial micro-/nanomotors that harvest environmental energy to move require energy surroundings; thus, their motion generally occurs in fuel solutions or under the real-time stimuli of external energy sources. Herein, inspired by vehicles, a refillable fuel-loading micromotor is proposed based on a 2 µm hemispherical multimetallic shell using catalase or platinum on its concave surface as the engine and the bowl structure as the fuel tank. H2O2 fuel is drawn into the microbowl by capillary action and restricted inside the bowl space through a self-generated O2 bubble cap on the microshell mouth. The periodic growth and burst of the O2 cap cause the enhanced diffusion motion of micromotors. This motion behavior can last for at least 30 min in a fuel-free environment with one H2O2 fueling. Additionally, the micromotor can be refilled repeatedly to achieve permanent motion. This demonstration of a refillable fuel-loading micromotor provides a model design of an energy built-in micromotor.

Enhanced 3D paper-based devices with a personal glucose meter for highly sensitive and portable biosensing of silver ion.

A variety of routine methods are available for the detection of silver (I) (Ag+) ions, but most of them rely on expensive, sophisticated and desktop instruments. Herein, a low-cost, instrument-free and portable Ag+ biosensor was described by initially designing a new class of 3D origami microfluidic paper-based analytical devices (µPADs) into each of which one piece of reagent-loaded nanoporous membrane was integrated. It combines analyte-triggered self-growing of silver nanoparticles to block the membrane's pores in situ for rapid yet efficient signal amplification with a handheld personal glucose meter for a portable and sensitive quantitative readout based on the biocatalytic reactions between the glucose oxidase and glucose. Its utility is well demonstrated with the specific detection of the analyte with a limit of detection as low as ∼58.1 pM (3σ), which makes this new biosensing method one of the most sensitive Ag+ assays in comparison with many other typical methods recently reported. Moreover, the satisfactory recovery of analyzing several types of real water examples, i.e., tap water, drinking water, pond water and soil water, additionally validates its feasibility for practical applications.

Equipment-Free Quantitative Aptamer-Based Colorimetric Assay Based on Target-Mediated Viscosity Change.

In this paper, we describe an aptamer-based colorimetric assay (ABCA), which integrates enzyme-loaded microparticles for signal amplification with distance measurement for equipment-free quantitative readout. The distance measurement readout is on the basis of target-induced selective reduction in viscosity of reaction solution. Its utility is well demonstrated with inexpensive, sensitive, and selective detection of adenosine (model analyte) in buffer samples and real samples of human serum and urine with the naked eye. This ABCA method just requires operators to simply count the number of colored distance-relevant marked bars on the calibrated glass microsyringes (testing containers) to provide quantitative results. It thus holds great promise for wide applications particularly in limited-resource settings.

Instrument-free quantitative gold nanoparticle-based liquid-phase colorimetric assays for use in resource-poor environments.

This work describes a new class of gold nanoparticle-based liquid-phase colorimetric assay (GNP-LPCA) termed as two dimensional (2D) GNP-LPCA. Its utility is demonstrated with the development of an aptamer-based 2D GNP-LPCA for simple, low-cost, sensitive, specific, and quantitative detection of adenosine as a model analyte in buffer and human serum samples with the naked eye.

Analyte-triggered autocatalytic amplification combined with gold nanoparticle probes for colorimetric detection of heavy-metal ions.

This work reports a new colorimetric nanosensor for the detection of heavy-metal ions that initially integrates analyte-triggered autocatalytic amplification with o-phenylenediamine-mediated aggregation of label-free gold nanoparticles. Its utility is well demonstrated with the simple, rapid, sensitive, and specific detection of Hg2+, Cu2+, and Ag+ targets with detection limits less than 3 nM.