Self-validating photothermal and electrochemical dual-mode sensing based on Hg2+ etching Ti3C2 MXene

Mercury ions can cause serious damage to the ecological environment, and it is necessary to develop reliable and elegant mercury ion sensors. In this protocol, a label-free photothermal/electrochemical dual-mode strategy for Hg2+ is proposed based on delaminated Ti3C2 MXene nanosheets (DL-Ti3C2 MXene). Hg2+ exists in water in the form of HgCl2, Hg(OH)2, and HgClOH, and the electron-rich elements O and Cl can specifically bind to the positively charged DL-Ti3C2 MXene at the edge, and further oxidation-reduction reaction occurs to obtain TiO2/C and Hg2Cl2. In view of the reduction activity and the performance of photothermal conversion of DL-Ti3C2 MXene itself, the electrochemical and photothermal responses decrease with the increase of the logarithm of Hg2+ concentration. The corresponding linear ranges are 50 pmol L-1-500 nmol L-1 and 1 nmol L-1-50 μmol L-1, and their detection limits calculated at 3 S/N are 17.2 pmol L-1 and 0.43 nmol L-1, respectively. DL-Ti3C2 MXene has the characteristics of a wide range of raw materials, low cost, and easy preparation. In addition, the design takes full advantage of the properties of the material itself, avoids the complex assembly and detection process of conventional sensors, and enables high selectivity and sensitivity for mercury detection. In particular, the dual-mode sensing endows self-confirmation of mercury ion detection results, thereby improving the reliability of the sensor.

Current Advances on the Single-Atom Nanozyme and Its Bioapplications

Nanozymes, a class of nanomaterials mimicking the function of enzymes, have aroused much attention as the candidate in diverse fields with the arbitrarily tunable features owing to the diversity of crystalline nanostructures, composition, and surface configurations. However, the uncertainty of their active sites and the lower intrinsic deficiencies of nanomaterial-initiated catalysis compared with the natural enzymes promote the pursuing of alternatives by imitating the biological active centers. Single-atom nanozymes (SAzymes) maximize the atom utilization with the well-defined structure, providing an important bridge to investigate mechanism and the relationship between structure and catalytic activity. They have risen as the new burgeoning alternative to the natural enzyme from in vitro bioanalytical tool to in vivo therapy owing to the flexible atomic engineering structure. Here, focus is mainly on the three parts. First, a detailed overview of single-atom catalyst synthesis strategies including bottom-up and top-down approaches is given. Then, according to the structural feature of single-atom nanocatalysts, the influence factors such as central metal atom, coordination number, heteroatom doping, and the metal-support interaction are discussed and the representative biological applications (including antibacterial/antiviral performance, cancer therapy, and biosensing) are highlighted. In the end, the future perspective and challenge facing are demonstrated.

Nanozyme's catalytic activity at neutral pH: reaction substrates and application in sensing

Nanozymes exhibit their great potential as alternatives to natural enzymes. In addition to catalytic activity, nanozymes also need to have biologically relevant catalytic reactions at physiological pH to fit in the definition of an enzyme and to achieve efficient analytical applications. Previous reviews in the nanozyme field mainly focused on the catalytic mechanisms, activity regulation, and types of catalytic reactions. In this paper, we discuss efforts made on the substrate-dependent catalytic activity of nanozymes at neutral pH. First, the discrepant catalytic activities for different substrates are compared, where the key differences are the characteristics of substrates and the adsorption of substrates by nanozymes at different pH. We then reviewed efforts to enhance reaction activity for model chromogenic substrates and strategies to engineer nanomaterials to accelerate reaction rates for other substrates at physiological pH. Finally, we also discussed methods to achieve efficient sensing applications at neutral pH using nanozymes. We believe that the nanozyme is catching up with enzymes rapidly in terms of reaction rates and reaction conditions. Designing nanozymes with specific catalysis for efficient sensing remains a challenge.

Enzyme-modulated photothermal immunoassay of chloramphenicol residues in milk and egg using a self-calibrated thermal imager

Highly sensitive and accurate detection of chloramphenicol is of paramount importance for food safety. Herein, an enzyme-modulated photothermal immunosensor that uses a self-calibrated thermal imaging system (SCTIS) as signal read-out was developed for detecting chloramphenicol. In this immunosensor, alkaline phosphatase was used as a modulator of the photothermal conversion. It could hydrolyze the substrate into ascorbic acid, thereby reducing oxidized 3,3',5,5'-tetramethylbenzidine, which exhibited a near-infrared laser-driven photothermal effect. For precise temperature measurement, the SCTIS was designed by using the temperature compensation of a ceramic chip to enable real-time self-calibration of the temperature. This SCTIS-based immunosensor could detect chloramphenicol with a LOD of 9 pg/mL in 2 h, and relative standard derivations from 3.95% to 13.58%. The average recoveries in milk and egg samples ranged from 76% to 114%. This versatile sensing strategy can detect various targets by altering recognition elements, thus has wide applicability in food safety testing and monitoring.

One-Pot Synthesis of MnOx-SiO2 Porous Composites as Nanozymes with ROS-Scavenging Properties

The development of nanomaterials that mimic the activity of enzymes is a topic of interest, for the decomposition of reactive oxygen species (ROS). We report the preparation of a novel nanocomposite of MnO_x needles covered with SiO₂ porous material. The material was prepared in one pot with a two-step procedure. The material was characterized by EDX, SEM, TEM, XRD, nitrogen adsorption-desorption isotherms, and XPS. The synthesis protocol took advantage of the atrane method, favoring the nucleation and initial growth of manganese oxide needles that remained embedded and homogeneously dispersed in a mesoporous silica matrix. The final composite had a high concentration of Mn (Si/Mn molar ratio of ca. 1). The nanozyme presented bimodal porosity: intraparticle and interparticle association with the surfactant micelles and the gaps between silica particles and MnO_x needles, respectively. The porosity favored the migration of the reagent to the surface of the catalytic MnO_x. The nanozyme showed very efficient SOD and catalase activities, thus improving other materials previously described. The kinetics were studied in detail, and the reaction mechanisms were proposed. It was shown that silica does not play an innocent role in the case of catalase activity, increasing the reaction rate.

Metalloporphyrin and gold nanoparticles modified hollow zeolite imidazole Framework-8 with excellent peroxidase like activity for quick colorimetric determination of choline in infant formula milk powder

Metalloporphyrin and gold nanoparticles (AuNPs) were fixed on the surface of hollow zeolite imidazole framework-8 (HZIF-8) by cation exchange and cross-linking reaction. The obtained hybrid nanozyme, Au/HZIF-8@TCPP(Fe), was fully characterized by TEM, HRTEM, EDS element mapping and XPS. Then, its peroxidase-like activity was explored with Km of 1.74 mM and Vmax of 9.60 × 10^-8 M·S^-1 towards H₂O₂, indicating excellent catalytic activity. Based on cascade reaction between choline oxidase and Au/HZIF-8@TCPP(Fe), a quick colorimetric method was established for choline detection in infant formula milk powder. After comprehensive verification, this method presented the merits of simple operation, satisfied detection limit (0.05 mM), wide linear range (0.05-2.0 mM), high accuracy (recovery of 92.2%-105.2%) and nice selectivity. This colorimetric method was applied to the determination of choline in milk powders of five brands. Our study could offer valuable reference for finding highly efficient nanozyme and constructing novel optical biosensors.

Tyndall-Effect-inspired assay with gold nanoparticles for the colorimetric discrimination and quantification of mercury ions and glutathione

This work initially reports a new nanosening method for simple, sensitive, specific, visual detection of mercury (II) (Hg²⁺) and glutathione (GSH) using the Tyndall Effect (TE) of the same colloidal gold nanoparticle (GNP) probes for efficient colorimetric signaling amplification. For the TE-inspired assay (TEA) method, arginine (Arg) molecules are pre-modified on the GNPs' surfaces (Arg-GNPs). Upon the Hg²⁺ introduction, it can be specifically coordinated with the terminal -NH₂ and -COOH groups of the Arg molecules to make the Arg-GNPs aggregate, producing a significantly-enhanced TE signal in the reaction solution after its irradiation by a 635-nm red laser pointer pen. On the other hand, the introduction of the GSH results in the production of the original Arg-GNPs' weak TE response, as it is able to bind such metal ion via mercury-thiol reactions to inhibit the above aggregation. Under the optimal conditions, the utility of the new TEA method is well demonstrated to quantitatively detect the Hg²⁺ and GSH with the aid of a smartphone as a portable TE reader during the linear concentration ranges of 50-3000 and 10-3000 nM, respectively. The detection limits for the Hg²⁺ and GSH are estimated to be as low as ∼3.5 and ∼0.3 nM, respectively. The recovery results obtained from the detection of Hg²⁺ in the complex tap and pond water samples and the assay of GSH in real human serum and urine samples are also satisfactory.

Integration and Quantitative Visualization of 3,3',5,5'-Tetramethylbenzidine-Probed Enzyme-Linked Immunosorbent Assay-like Signals in a Photothermal Bar-Chart Microfluidic Chip for Multiplexed Immunosensing

The photothermal effect shows significant promise for various biomedical applications but is rarely exploited for microfluidic lab-on-a-chip bioassays. Herein, a photothermal bar-chart microfluidic immunosensing chip, with the integration of the conventional 3,3',5,5'-tetramethylbenzidine (TMB)-probed enzyme-linked immunosorbent assay (ELISA)-like system, was developed based on exploiting the photothermal pumping technique for visual bar-chart microfluidic immunosensing. Both the sandwich ELISA-like system and the photothermal pumping protocol were integrated into a single photothermal bar-chart chip. On-chip immunocaptured iron oxide nanoparticles catalyzed the oxidation of the chromogenic substrate, TMB, to produce a sensitive photothermal and chromogenic dual-functional probe, oxidized TMB. As the result of heat generation and the subsequent production of elevating vapor pressure in the sealed microfluidic environment, the on-chip near-infrared laser-driven photothermal effect of the probe served as a dose-dependent pumping force to drive the multiplexed quantitative display of the immunosensing signals as visual dye bar charts. Prostate-specific antigen as a model analyte was tested at a limit of detection of 1.9 ng·mL^-1, lower than the clinical diagnostic threshold of prostate cancer. This work presents a new perspective for microfluidic integration and multiplexed quantitative bar-chart visualization of the conventional TMB-probed ELISA signals possibly by means of an affordable handheld laser pointer in a lab-on-a-chip format.

A portable thermal detection method based on the target responsive hydrogel mediated self-heating of a warming pad

A simple thermal aptasensing platform was devised for the sensitive detection of organophosphate pesticides (using malathion as a model target) based on the efficient self-heating reaction of a warming pad with a switchable target responsive enzyme-encapsulated three-dimensional (3D) DNA hydrogel using a portable thermometer as a signal readout in this work. The existence of the target malathion would open the catalase-3D network and lots of catalase was released from the hydrogel, which could efficiently convert H₂O₂ to an O₂ molecule. The product O₂ is the critical condition for the self-heating of the warming pad. Thereafter, the temperature was enhanced with the increasing amount of O₂. The strategy displays outstanding specificity, reproducibility and stability. Moreover, this method can be easily extended to monitor other molecules using different aptamer sequences in practical applications.

Integration of organic and inorganic photothermal probes for enhanced photothermometric sensing of silver ions

A new signal-amplified photothermometric sensor of Ag⁺ was explored based on a simple yet effective integration of inorganic and organic photothermal probes, mainly depending on the successful exploitation of a dual-signal transduction channel originating from the inherent photothermal property and the peroxidase-like activity of Prussian blue nanocubes (PB NCs).

Fe(III)-mediated reversible catalytic activity of MoS2 nanozymes for bisphosphonate drug sensing

Peroxidase-like activity of MoS₂ quantum dots (QDs) can be reversibly regulated by means of Fe³⁺/alendronate sodium (ALDS)-induced aggregation/disaggregation of the QDs in solution. Specifically, Fe³⁺ can selectively aggregate the MoS₂ QDs and thus greatly enhance their peroxidase-like activity, while such enhancement can be inhibited in the presence of ALDS owing to the competitive coordination of ALDS with Fe³⁺. By regulating the enzyme-like activity of MoS₂ QDs, different colorimetric signal of a typical substrate of horseradish peroxidase, 3,3΄,5,5΄-tetramethylbenzidine, can be measured in the presence of H₂O₂. Based on this mechanism, we develop a colorimetric approach for the determination of ALDS and further applied in quality control of pharmaceutical products, utilizing either smartphone or UV-vis spectrometer as a readout. This detection method is rapid and selective, where derivatization of ALDS before detection is not needed. Such a smartphone-based colorimetric detection platform is promising to be applied in point-of-care testing at home, small clinics, or underdeveloped regions.

A competitive colorimetric aptasensor transduced by hybridization chain reaction-facilitated catalysis of AuNPs nanozyme for highly sensitive detection of saxitoxin

Saxitoxin (STX) is a small molecule toxin (Mw. ca. 299 g/mol) with high acute toxicity, and it has urgent need of facile analytical methods. Herein, a competitive colorimetric aptasensor was developed for highly sensitive detection of STX. An anti-STX aptamer was hybridized with a complementary strand on the magnetic beads and was competitively bound by STX. The supernatant containing the aptamer binding to STX was obtained by magnetic separation, which could trigger hybridization chain reaction (HCR) to generate rigid double stranded DNAs (dsDNAs) with sticky end and variable length. These HCR-dsDNAs were found to be able to facilitate significant enhancement on the peroxidase-like catalytic capability of AuNPs nanozyme towards 3,3,5,5-tetramethylbenzidine (TMB). The concentration of STX was responded in a "turn on" mode, based on the amplified colorimetric transduction thereof. The aptasensor realized high sensitivity, with a limit of detection (LOD) as low as 42.46 pM. Moreover, a wide linear detection range of 78.13-2500 pM, good selectivity, as well as good recovery rates of 106.2-113.5% when analyzing STX in real shellfish samples were obtained. This strategy could be referred to develop robust aptasensors for simple and highly sensitive detection of other small molecules and toxins.

Photothermometric analysis of bismuth ions using aggregation-induced nanozyme system with a target-triggered surface cleaning effect

The development of nanozyme-based photothermometric sensing for point-of-care testing (POCT) heavy metal ions is of great significance for disease diagnosis and health management. Considering the low catalytic activity of most nanozymes at physiological pH, we found bismuth ions (Bi³⁺) could effectively enhance the peroxidase (POX)-like activity of cetyltrimethylammonium bromide and citrate-capped octahedral gold nanoparticle (CTAB/Cit-AuNP) nanozymes. It is mainly based on Bi³⁺ ions being able to trigger the surface cleaning effect of CTAB/Cit-AuNPs. Because the more active Bi³⁺ ions could effectively bind with citrate on the gold surface and competitively destroy the electrostatic interaction between citrate and CTAB, resulting in the removal of CTAB ligands from the gold surface. Without the ligand protection, CTAB/Cit-AuNPs aggregated immediately, and further resulted in a significant activation of the POX-like activity of AuNP nanozymes. Based on this principle, we introduced the enzyme substrate 3,3',5,5'-tetramethylbenzidine (TMB) into this aggregation-induced nanozyme system, and rationally designed a photothermometric platform to quickly and sensitively detect Bi³⁺ ions by using the good photothermal effect of the oxidation product of TMB (oxTMB). The developed photothermometric method only using a common thermometer has a limit of detection (LOD) as low as 45.7 nM for POCT analysis of Bi³⁺ ions. This study not only provides a more accurate understanding of the aggregation-induced nanozymes based on the surface cleaning principle, but also shows the potential applications of aggregation-induced nanozymes in the POCT field.

Ultrasensitive fluorometric biosensor based on Ti3C2 MXenes with Hg2+-triggered exonuclease III-assisted recycling amplification

Herein a fluorometric bioanalysis platform for mercury(ii) (Hg²⁺) detection was developed using two-dimensional MXenes (Ti₃C₂) as fluorescence quencher and Hg²⁺-induced exonuclease III-assisted target recycling strategy for efficient signal amplification.