Lanthanide-Boosted Singlet Oxygen from Diverse Photosensitizers along with Potent Photocatalytic Oxidation

Highly active nanozyme based on nitrogen-doped graphene quantum dots and iron ion nanocomposite for selective colorimetric detection of hydroquinone

Inspired by the iron porphyrin structure of natural horseradish peroxidase (HRP), an efficient carbon-based nanozyme was fabricated using nitrogen-doped graphene quantum dots (NGQDs) and iron ion (Fe3+) nanocomposite, enabling selective distinguishment of hydroquinone (HQ) from its isomers. NGQDs with good dispersibility and uniform size were synthesized via a one-step hydrothermal process. NGQDs lacked peroxidase-like activity but the formed nanocomposite (Fe3+-NGQDs) upon Fe3+ addition possessed high peroxidase-like activity. Fe3+-NGQDs nanocomposite exhibited shuttle-shaped structure (∼30 nm), the lattice structure of NGQDs and electron transfer between Fe3+ and NGQDs. The Fe3+-NGQDs nanocomposite can catalyze the production of superoxide radicals (•O2-) from H2O2. The Michaelis constant (Km) of Fe3+-NGQDs (0.115 mM) was lower than that of natural HRP (0.434 mM) with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate and the maximum initial reaction rate (Vmax, 16.47 × 10-8 M/s) was nearly 4 times higher than that of HRP using H2O2 substrate. HQ, unlike its isomers catechol (CC) and resorcinol (RE), could consume •O2- generated from the decomposition of H2O2 catalyzed by Fe3+-NGQDs nanocomposite, reducing the oxidation of TMB. This principle enabled selective colorimetric determination of HQ ranged from 1 μM to 70 μM and a limit of detection (LOD) of 0.2 μM. Successful determination of HQ in pond water was also realized.

Dielectric barrier discharge-induced chemical vapor generation for highly sensitive analysis of iodine in seawater by ICP-MS

BACKGROUND

The accurate determination of iodine in seawater is essential to understanding its impact on the environment and human health. Inductively coupled plasma mass spectrometry (ICP-MS) is a widely used technique for elemental analysis due to its high sensitivity, speed, and low limit of detection (LOD). However, its capability in the detection of iodine in complex matrix samples is still limited by the low sample introduction efficiency of pneumatic nebulization and the high ionization energy of iodine. Dielectric barrier discharge microplasma-induced vapor generation (DBD-μPIVG) is a sample introduction technique that has been widely coupled with atomic spectrometry due to its high vapor generation efficiency, rapid reaction speed, high anti-interference capability, and environmental friendliness.

RESULTS

A new method was developed for the rapid and sensitive determination of iodine using DBD-μPIVG coupled with ICP-MS. The DBD-μPIVG sample introduction technique can convert both iodide and iodate to their volatiles with a vapor generation efficiency of 70 %. The experimental conditions were optimized in detail, and the LOD for iodine was 0.04 μg L-1, which was lower compared to pneumatic nebulization and comparable to that after the extraction treatment. The relative standard deviation (RSD) obtained after 11 replicate determinations was 2.4%. Furthermore, the potential mechanism and anti-interference performance of the proposed method were also carefully investigated.

SIGNIFICANCE

Compared to other analytical methods for iodine analysis, this approach is environmentally friendly, exhibits high anti-interference capability and enables accurate determination of iodine in complex matrix samples. The high vapor generation efficiency of DBD-μPIVG improves the sensitivity for iodine detection and expands the applicable elemental range of DBD-μPIVG. Finally, the proposed method was successfully applied to analyze the iodine content in seawater samples obtained from the Chinese coastal waters and retains great potential for assessing the distribution of iodine in different sea areas.

Breaking the pH Limitation of Nanozymes: Mechanisms, Methods, and Applications

Although nanozymes have drawn great attention over the past decade, the activities of peroxidase-like, oxidase-like, and catalase-like nanozymes are often pH dependent with elusive mechanism, which largely restricts their application. Therefore, a systematical discussion on the pH-related catalytic mechanisms of nanozymes together with the methods to overcome this limitation is in need. In this review, various nanozymes exhibiting pH-dependent catalytic activities are collected and the root causes for their pH dependence are comprehensively analyzed. Subsequently, regulatory concepts including catalytic environment reconstruction and direct catalytic activity improvement to break this pH restriction are summarized. Moreover, applications of pH-independent nanozymes in sensing, disease therapy, and pollutant degradation are overviewed. Finally, current challenges and future opportunities on the development of pH-independent nanozymes are suggested. It is anticipated that this review will promote the further design of pH-independent nanozymes and broaden their application range with higher efficiency.

Modifying Parallel Excitations into One Framework for C(sp3)─H Bond Activation with Energy Combined More Than Two Photons

Natural photosynthesis enzymes utilize energies of several photons for challenging oxidation of water, whereas artificial photo-catalysis typically involves only single-photon excitation. Herein, a multiphoton excitation strategy is reported that combines parallel photo-excitations with a photoinduced electron transfer process for the activation of C(sp3)─H bonds, including methane. The metal-organic framework Fe3-MOF is designed to consolidate 4,4',4″-nitrilotrisbenzoic units for the photoactivation of dioxygen and trinuclear iron clusters as the HAT precursor for photoactivating alkanes. Under visible light irradiation, the dyes and iron clusters absorbed parallel photons simultaneously to reach their excited states, respectively, generating 1O2 via energy transfer and chlorine radical via ligand-to-metal charge transfer. The further excitation of organic dyes leads to the reduction of 1O2 into O2 •- through a photoinduced electron transfer, guaranteeing an extra multiphoton oxygen activation manner. The chlorine radical abstracts a hydrogen atom from alkanes, generating the carbon radical for further oxidation transformation. Accordingly, the total oxidation conversion of alkane utilizing three photoexcitation processes combines the energies of more than two photons. This new platform synergistically combines a consecutive excited photoredox organic dye and a HAT catalyst to combine the energies of more than two photons, providing a promising multiphoton catalysis strategy under energy saving, and high efficiency.

Field Detection of Uranyl in Coastal Water of China Using a Portable Device via DNA Photocleavage

The urgent need for field detection of uranium in seawater is 2-fold: to provide prompt guidance for uranium extraction and to prevent human exposure to nuclear radiation. However, current methods for this purpose are largely hindered by bulky instrumentation, high costs of developed materials, and severe matrix interferences, which limit their further application in the field. Herein, we demonstrated a portable and label-free strategy for the field detection of uranyl in seawater based on the efficient photocleavage of DNA. Further experiments confirmed the generation of ultraviolet (UV) light-induced reactive oxygen species (ROS), such as O2•- and •OH, which fragmented oligomeric DNA in the presence of uranyl and UV light. Detailed studies showed that DNA significantly enhances uranyl absorption in the UV-visible region, leading to the generation of more ROS. A fluorescence system for the selective detection of uranyl in seawater was established by immobilizing two complementary oligonucleotides with the fluorescent dye SYBR Green I. The strategy of UV-induced photocleavage offers high selectivity, excellent interference immunity, and high sensitivity for uranyl, with a detection limit of 6.8 nM. Additionally, the fluorescence can be visually detected using a 3D-printed miniaturized device integrated with a smartphone. This method has been successfully applied to the on-site detection of uranyl in seawater in 18 Chinese coastal cities and along the coast of Hainan Island within 3 min for a single sample. The sample testing and field analysis results indicate that this strategy has promising potential for real-time monitoring of trace uranyl in China's coastal waters. It is expected to be utilized for the rapid assessment of nuclear contamination and nuclear engineering construction.

Ultrasmall CuMn-His Nanozymes with Multienzyme Activity at Neutral pH: Construction of a Colorimetric Sensing Array for Biothiol Detection and Disease Identification

Biothiol assays offer vital insights into health assessment and facilitate the early detection of potential health issues, thereby enabling timely and effective interventions. In this study, we developed ultrasmall CuMn-Histidine (His) nanozymes with multiple enzymatic activities. CuMn-His enhanced peroxidase (POD)-like activity at neutral pH was achieved through hydrogen bonding and electrostatic effects. In addition, CuMn-His possesses laccase (LAC)-like and superoxide dismutase (SOD)-like activities at neutral pH. Based on three different enzyme mimetic activities of CuMn-His at neutral pH, the colorimetric sensing array without changing the buffer solution was successfully constructed. The array was successfully used for the identification of three biothiols, glutathione (GSH), cysteine (Cys), and homocysteine (Hcy). Subsequently, excellent application results were shown in complex serum and cellular level analyses. This study provides an innovative strategy for the development of ultrasmall bimetallic nanozymes with multiple enzymatic activities and the construction of colorimetric sensing arrays.

Light-activated, dual-mode fluorescence and colorimetric detection of estradiol with high fidelity based on aptamer's special recognition

The intake of estradiol residue from food will lead to health problems, so the rapid and reliable detection of estradiol residue is essential. Multi-mode assays are inherently self-correcting and self-validating, providing more reliable, interference-resistant, high-fidelity results. Here, we developed a dual-mode method to achieve a rapid, reliable, and sensitive detection of estradiol. The binding of thioflavin T to the cavity sites of estradiol aptamer not only generates a strong fluorescence signal, but also provides light-activated oxidase activity to produce a blue oxidation product. But the specific binding between aptamer and estradiol will compete with the above process. Thus a dual-mode fluorescence and colorimetric detection of estradiol was realized with a detection limit of 0.15 μM or 0.27 μM. Besides, the dual-mode method showed a good selectivity for estradiol detection. The method could detect estradiol in meat food, showing a good application potential to monitor endocrine-disrupting chemical.

Sustainable, efficient, and synergistic photocatalytic degradation toward organic dyes and formaldehyde gas via Cu2O NPs@wood

Cuprous oxide (Cu2O) nanoparticles (NPs) was anchored on wood by simple spraying method, then both soft and hard wood has been endowed efficient function photocatalytic degradation toward organic dyes and formaldehyde gas synergistically. The best recycle ability of wood based photocatalyst toward organic pollutants was achieved, which was characterized by photocatalytic degradation efficiency of methylene blue (MB) more than 95% after 100 cycles, and formaldehyde gas over 85% after 60 cycles. Cu2O NPs@wood performed much lower forbidden bandwidth (Eg), which accelerated to generate much more radical of e- and finally promoted the capacity of photocatalytic degradation. The proposed Cu2O NPs@wood catalysts has potential to be applied both in the field of wastewater and air pollution remediation.

Singlet oxygen: Properties, generation, detection, and environmental applications

Singlet oxygen (1O2) is molecular oxygen in the excited state with high energy and electrophilic properties. It is widely found in nature, and its important role is gradually extending from chemical syntheses and medical techniques to environmental remediation. However, there exist ambiguities and controversies regarding detection methods, generation pathways, and reaction mechanisms which have hindered the understanding and applications of 1O2. For example, the inaccurate detection of 1O2 has led to an overestimation of its role in pollutant degradation. The difficulty in detecting multiple intermediate species obscures the mechanism of 1O2 production. The applications of 1O2 in environmental remediation have also not been comprehensively commented on. To fill these knowledge gaps, this paper systematically discussed the properties and generation of 1O2, reviewed the state-of-the-art detection methods for 1O2 and long-standing controversies in the catalytic systems. Future opportunities and challenges were also discussed regarding the applications of 1O2 in the degradation of pollutants dissolved in water and volatilized in the atmosphere, the disinfection of drinking water, the gas/solid sterilization, and the self-cleaning of filter membranes. This review is expected to provide a better understanding of 1O2-based advanced oxidation processes and practical applications in the environmental protection of 1O2.

Acid-induced tubular g-C3N4 for the selective generation of singlet oxygen by energy transfer: Implications for the photocatalytic degradation of parabens in real water environments

Parabens are widely present in aquatic environments and pose potential health risk. Although great progress has been made in the field of the photocatalytic degradation of parabens, the powerful Coulomb interactions between electrons and holes are the major limitations to photocatalytic performance. Hence, acid-induced tubular g-C3N4 (AcTCN) was prepared and applied for the removal of parabens from a real water environment. AcTCN not only increased the specific surface area and light absorption capacity, but also selectively generated 1O2 via an energy transfer-mediated oxygen activation pathway. The 1O2 yield of AcTCN was 11.8 times higher than that of g-C3N4. AcTCN exhibited remarkable removal efficiencies for parabens depending on the length of the alkyl group. Furthermore, the rate constants (k values) of parabens in ultrapure water were higher than those in tap and river water because of the presence of organic and inorganic species in real water environments. Two possible pathways for the photocatalytic degradation of parabens are proposed based on the identification of intermediates and theoretical calculations. In summary, this study offers theoretical support for the efficient enhancement of the photocatalytic performance of g-C3N4 for the removal of parabens in real water environments.

A comprehensive exploration of the latest innovations for advancements in enhancing selectivity of nanozymes for theranostic nanoplatforms

Nanozymes have captured significant attention as a versatile and promising alternative to natural enzymes in catalytic applications, with wide-ranging implications for both diagnosis and therapy. However, the limited selectivity exhibited by many nanozymes presents challenges to their efficacy in diagnosis and raises concerns regarding their impact on the progression of disease treatments. In this article, we explore the latest innovations aimed at enhancing the selectivity of nanozymes, thereby expanding their applications in theranostic nanoplatforms. We place paramount importance on the critical development of highly selective nanozymes and present innovative strategies that have yielded remarkable outcomes in augmenting selectivities. The strategies encompass enhancements in analyte selectivity by incorporating recognition units, refining activity selectivity through the meticulous control of structural and elemental composition, integrating synergistic materials, fabricating selective nanomaterials, and comprehensively fine-tuning selectivity via approaches such as surface modification, cascade nanozyme systems, and manipulation of external stimuli. Additionally, we propose optimized approaches to propel the further advancement of these tailored nanozymes while considering the limitations associated with existing techniques. Our ultimate objective is to present a comprehensive solution that effectively addresses the limitations attributed to non-selective nanozymes, thus unlocking the full potential of these catalytic systems in the realm of theranostics.

Engineering a gold nanoparticles-carbon dots nanocomposite with pH-flexibility for monitoring hydrogen peroxide released from living cells

Constructing nanozymes with satisfactory catalytic efficiency under physiological conditions is still in great demand for facilitating the advancement of biocatalysts. We herein present a gold nanoparticles-carbon dots nanocomposite (Au-CDs) as an efficient photo-activated nanozyme for monitoring H2O2 released from living cells. The integration of CDs with AuNPs remarkably accelerates the catalytic activity at neutral pH via engaging Mn3+ ions as the mediators. Meanwhile, the reserved cyclodextrin cavities also enhance the adsorption capacity towards chromogenic substrates through host-guest interactions. Moreover, taking advantage of the inhibitory effect of H2O2 on the photo-oxidation ability of the Au-CDs nanocomposite, the Au-CDs based colorimetric method was able to realize in situ assessment of the hydrogen peroxide (H2O2) released from living cells. This method paves a new way to establish a promising biosensing platform for unraveling biological events.

Light-activated carbon dot nanozyme with scandium for a highly efficient and pH-universal bio-nanozyme cascade colorimetric assay

Nanozyme-based colorimetric assays have attracted much attention due to their cost-effectiveness, high stability, and sensitivity. In particular, the catalytic cascade imparted by the biological enzyme is highly selective. However, developing an efficient, one-pot, and pH-universal bio-nanozyme cascade remains challenging. Considering the tunable activity of the photo-activated nanozyme, we herein demonstrated a pH-universal colorimetric assay based on the Sc³⁺-boosted photocatalytic oxidation of carbon dots (C-dots). As a strong Lewis acid, Sc³⁺ shows ultra-fast complexation with OH^- over a broad pH range and dramatically decreases the pH of the buffer solutions. In addition to regulating the pH, Sc³⁺ also binds to the C-dots to produce a persistent and strongly oxidizing intermediate based on photo-induced electron transfer. The proposed Sc³⁺-boosted photocatalytic system was successfully used in a cascade colorimetric assay with biological enzymes for assessing their activity as well as the detection of enzyme inhibitors at neutral and alkaline pH. Instead of designing new nanozymes for catalytic cascades, this work suggests that introducing promoters can be a convenient strategy in practical applications.

Light-Elicited and Oxygen-Saved Iridium Nanocapsule for Oxidative Damage Intensified Oncotherapy

Regulating redox homeostasis in tumor cells and exploiting oxidative stress to damage tumors is an efficacious strategy for cancer therapy. However, the strengths of organic nanomaterials within this strategy are often ignored. In this work, a light-triggered reactive oxygen species (ROS) damaging nanoamplifier (IrP-T) was developed for enhanced photodynamic therapy (PDT). The IrP-T was fabricated with an amphiphilic iridium complex and a MTH1 inhibitor (TH287). Under green light stimulation, IrP-T catalyzed the oxygen in cells to generate ROS for realizing oxidative damage; meanwhile, TH287 increased the accumulation of 8-oxo-dGTP, further strengthening oxidative stress and inducing cell death. IrP-T could maximize the use of a small amount of oxygen, thus further boosting the efficacy of PDT in hypoxic tumors. The construction of nanocapsules provided a valuable therapeutic strategy for oxidative damage and synergizing PDT.

Adaptable graphitic C6N6-based copper single-atom catalyst for intelligent biosensing

Self-adaptability is highly envisioned for artificial devices such as robots with chemical noses. For this goal, seeking catalysts with multiple and modulable reaction pathways is promising but generally hampered by inconsistent reaction conditions and negative internal interferences. Herein, we report an adaptable graphitic C₆N₆-based copper single-atom catalyst. It drives the basic oxidation of peroxidase substrates by a bound copper-oxo pathway, and undertakes a second gain reaction triggered by light via a free hydroxyl radical pathway. Such multiformity of reactive oxygen-related intermediates for the same oxidation reaction makes the reaction conditions capable to be the same. Moreover, the unique topological structure of Cu_SAC₆N₆ along with the specialized donor-π-acceptor linker promotes intramolecular charge separation and migration, thus inhibiting negative interferences of the above two reaction pathways. As a result, a sound basic activity and a superb gain of up to 3.6 times under household lights are observed, superior to that of the controls, including peroxidase-like catalysts, photocatalysts, or their mixtures. Cu_SAC₆N₆ is further applied to a glucose biosensor, which can intelligently switch sensitivity and linear detection range in vitro.

Iodine-mediated photoinduced autoinductive tandem chromogenic system for visual colorimetric detection of triacetone triperoxide explosive

A long-standing challenge in colorimetric detection of triacetone triperoxide (TATP) explosive is low sensitivity. We herein developed an iodine-mediated photoinduced auto-inductive tandem chromogenic system to achieve exponential signal amplification. The strategy employs the KI-TATP reaction and photo-induced autocatalytical oxidation of o-phenylenediamine (OPD) that work in tandem. The resulting I₃^- from the KI-TATP reaction oxidizes OPD to yellow 2,3-diaminophenazine (DAP) that is further excited by blue light illumination to produce reactive oxygen species (ROS). The obtained ROS, in turn, promotes the oxidation of OPD to gain more DAP, causing the auto-inductive chromogenic reaction processes. This tandem chromogenic system is applied for visual colorimetric detection of TATP, allowing the selective and sensitive detection of TATP down to 42.8 μM. Moreover, analyses of TATP in real samples are performed, and the satisfactory recovery results are achieved. Furthermore, a field detection kit is also developed.

Applications of Fluorescent Carbon Dots as Photocatalysts: A Review

Carbon dots (CDs) have attracted considerable interest from the scientific community due to their exceptional properties, such as high photoluminescence, broadband absorption, low toxicity, water solubility and (photo)chemical stability. As a result, they have been applied in several fields, such as sensing, bioimaging, artificial lighting and catalysis. In particular, CDs may act as sole photocatalysts or as part of photocatalytic nanocomposites. This study aims to provide a comprehensive review on the use of CDs as sole photocatalysts in the areas of hydrogen production via water splitting, photodegradation of organic pollutants and photoreduction and metal removal from wastewaters. Furthermore, key limitations preventing a wider use of CDs as photocatalysts are pointed out. It is our hope that this review will serve as a basis on which researchers may find useful information to develop sustainable methodologies for the synthesis and use of photocatalytic CDs.

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.

NIR-II Fluorescence Imaging-Guided Oxygen Self-Sufficient Nano-Platform for Precise Enhanced Photodynamic Therapy

Tumor hypoxia and systemic toxicity seriously affect the efficacy of photodynamic therapy (PDT) and are considered as the "Achilles' heel" of PDT. Herein, to combat such limitations, an intelligent orthogonal emissions LDNP@SiO₂ -CaO₂ and folic acid-polyethylene glycol-Ce6 nanodrug is rationally designed and fabricated not only for relieving the hypoxic tumor microenvironment (TME) to enhance PDT efficacy, but also for determining the optimal triggering time through second near-infrared (NIR-II) fluorescence imaging. The designed nanodrug continuously releases a large amount of O₂ , H₂ O₂ , and Ca²⁺ ions when exposed to the acidic TME. Meanwhile, under downshifting NIR-II bioimaging guidance, chlorine e6 (Ce6) consumes oxygen to produce ¹ O₂ upon excitation of upconversion photon. Moreover, cytotoxic reactive oxygen species (ROS) and calcium overload can induce mitochondria injury and thus enhance the oxidative stress in tumor cells. As a result, the NIR-II bioimaging guided TME-responsive oxygen self-sufficient PDT nanosystem presents enhanced anti-tumor efficacy without obvious systemic toxicity. Thus, the fabricated nanodrug offers great potential for designing an accurate cancer theranostic system.

Protein-Mimicking Nanoparticles in Biosystems

Proteins are essential elements for almost all life activities. The emergence of nanotechnology offers innovative strategies to create a diversity of nanoparticles (NPs) with intrinsic capacities of mimicking the functions of proteins. These artificial mimics are produced in a cost-efficient and controllable manner, with their protein-mimicking performances comparable or superior to those of natural proteins. Moreover, they can be endowed with additional functionalities that are absent in natural proteins, such as cargo loading, active targeting, membrane penetrating, and multistimuli responding. Therefore, protein-mimicking NPs have been utilized more and more often in biosystems for a wide range of applications including detection, imaging, diagnosis, and therapy. To highlight recent progress in this broad field, herein, representative protein-mimicking NPs that fall into one of the four distinct categories are summarized: mimics of enzymes (nanozymes), mimics of fluorescent proteins, NPs with high affinity binding to specific proteins or DNA sequences, and mimics of protein scaffolds. This review covers their subclassifications, characteristic features, functioning mechanisms, as well as the extensive exploitation of their great potential for biological and biomedical purposes. Finally, the challenges and prospects in future development of protein-mimicking NPs are discussed.

Visible light-driven i-motif-based DNAzymes

DNA foldings provide variant possibilities to develop DNAzymes with remarkable catalytic performance. In spite of fruitful reports on G-quadruplex DNAzymes, four-stranded cytosine-rich i-motifs have not been explored as the potential skeletons of DNAzymes. In this work, we developed a visible light-driven DNAzyme based on human telomeric i-motifs using a natural photosensitizer of hypericin (Hyp) as the cofactor and dissolved oxygen as the oxidant source. The i-motif folding in acidic solution caused the distal thymine overhangs at the 3' and 5' ends to approach each other to provide a favorable binding site for Hyp via an interaction of fully complementary hydrogen bonding. However, the i-motifs without the distal overhangs or with the inappropriate overhang length and the base identity exhibited no binding with Hyp. The binding event converted Hyp from the fully dark state to the emissive state under visible light illumination. Subsequently, the excited Hyp had an opportunity to transfer energy to dissolved oxygen. Resultantly, singlet oxygen (¹O₂) was generated to initiate the substrate oxidation. The catalytic performance of the DNAzyme can be improved using a long-lived mediator. Our developed i-motif-based DNAzyme can be driven by almost the whole range of visible lights, suggesting broad applications in the photocatalytic fields, for example, as an alternative strategy in developing biodevices.

Modulating the catalytic activity of gold nanoparticles using amine-terminated ligands

Nanozymes have broad applications in theranostics and point-of-care tests. To enhance the catalytic activity of nanozymes, the conventional strategy is doping metals to form highly active nanoalloys. However, high-quality and stable nanoalloys are hard to synthesize. Ligand modification is a powerful strategy to achieve chemoselectivity or bioactivity by changing the surface chemistry. Here, we explore different ligands to enhance the catalytic activity of nanozymes, e.g., gold nanoparticles (AuNPs). We systematically studied the impacts on the enzymatic activity of AuNPs by ligand engineering of surface chemistry (charge, group, and surface distance). Our work established critical guidelines for surface modification of nanozymes. The amine group favors higher activity of AuNPs than other groups. The flexible amine-rich ligand enhances the catalytic activity of AuNPs in contrast to other ligands and unmodified AuNPs. Using a proof-of-concept model, we screened many candidate ligands to obtain polyamine-AuNPs, which have strongly enhanced peroxidase-like activity and 100 times enhanced sensitivity compared to unmodified AuNPs. The strategy of enhancing the catalytic activity of AuNPs using ligands will facilitate the catalysis-related applications of nanozymes in biology and diagnostics.

Surface ligand engineering can precisely modulate the catalytic activity of nanozymes from inactive to highly active.

Activating the Neutral pH Photozymatic Activity of g-C3N4 Nanosheet through Post-Synthetic Incorporation of Pt

The catalytic activity of photozyme can be regulated through light irradiation time and intensity, but it still suffers from low activity in physiological neutral pH (typically, pH < 5). Herein,...

Colorimetric and Fluorescent Dual-Modality Sensing Platform Based on Fluorescent Nanozyme

Compared with natural enzymes, nanozymes based on carbonaceous nanomaterials are advantages due to high stability, good biocompatibility, and the possibility of multifunctionalities through materials engineering at an atomic level. Herein, we present a sensing platform using a nitrogen-doped graphene quantum dot (NGQD) as a highly efficient fluorescent peroxidase mimic, which enables a colorimetric/fluorescent dual-modality platform for detection of hydrogen peroxide (H₂O₂) and biomolecules (ascorbic acid-AA, acid phosphatase-ACP) with high sensitivity. NGQD is synthesized using a simple hydrothermal process, which has advantages of high production yield and potential for large-scale preparation. NGQD with uniform size (3.0 ± 0.6 nm) and a single-layer graphene structure exhibits bright and stable fluorescence. N-doping and ultrasmall size endow NGQD with high peroxidase-mimicking activity with an obviously reduced Michaelis–Menten constant (K_m) in comparison with natural horseradish peroxidase. Taking advantages of both high nanozyme activity and unique fluorescence property of NGQD, a colorimetric and fluorescent dual-modality platform capable of detecting H₂O₂ and biomolecules (AA, ACP) with high sensitivity is developed as the proof-of-concept demonstration. Furthermore, the mechanisms underlying the nanozyme activity and biosensing are investigated.

A Metal-Organic Framework as a Multiphoton Excitation Regulator for the Activation of Inert C(sp3 )-H Bonds and Oxygen

The activation and oxidization of inert C(sp³ )-H bonds into value-added chemicals affords attractively economic and ecological benefits as well as central challenge in modern chemistry. Inspired by the natural enzymatic transformation, herein, we report a new multiphoton excitation approach to activate the inert C(sp³ )-H bonds and oxygen by integrating the photoinduced electron transfer (PET), ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) events together into one metal-organic framework. The well-modified nicotinamide adenine dinucleotide (NAD⁺ ) mimics oxidized Ce^III -OEt moieties to generate Ce^IV -OEt chromophore and its reduced state mimics NAD^. via PET. The in situ formed Ce^IV -OEt moiety triggers a LMCT excitation to form the alkoxy radical EtO^. , abstracts a hydrogen atom from the C(sp³ )-H bond, accompanying the recovery of Ce^III -OEt and the formation of alkyl radicals. The formed NAD^. activates oxygen to regenerate the NAD⁺ for next recycle, wherein, the activated oxygen species interacts with the intermediates for the oxidization functionalization, paving a catalytic avenue for developing scalable and sustainable synthetic strategy.

Dual-Emitting Carbonized Polymer Dots Synthesized at Room Temperature for Ratiometric Fluorescence Sensing of Vitamin B12

Ratiometric fluorescence (FL) probes are highly desirable for highly sensitive and reliable assays. Dual-emitting carbonized polymer dots (CPDs) have great application prospects in building ratiometric FL sensors. However, dual-emitting CPDs are usually synthesized at high temperatures and high pressures, which not only increases the cost but also complicates the structure of CPDs. Here, we developed a facile strategy for the fabrication of dual-emitting CPDs at room temperature using tetrachlorobenzoquinone and ethylenediamine. The formation of CPDs was induced by Schiff base condensation reaction, enabling the following cross-linking polymerization process. The dual-emitting CPDs demonstrate good photostability and antioxidant capacity. Importantly, the typical dual-emission bands of the as-prepared CPDs are found to have a blue emission band at 445 nm with a maximum excitation of 350 nm and a yellow emission band at 575 nm with a maximum excitation of 440 nm. Based on the dual-emitting property of CPDs, a ratiometric FL nanoprobe is obtained for sensitive determination of vitamin B12 (VB12), as the inner filtering and static quenching effects between VB12 and CPDs allow effective quenching of the blue FL of CPDs, while the yellow FL is maintained. The established assay shows linear detection ranges of 0.25-100 μM with a low limit of detection of 0.14 μM. These findings provide new guidance for the facile preparation of CPDs with excellent dual-emitting optical properties, indicating good prospects in biosensing.

In Situ Fabrication of Nanoceria with Oxidase-like Activity at Neutral pH: Mechanism and Boosted Bio-Nanozyme Cascades

Catalytic cascades have drawn much attention by avoiding the isolation of intermediates and due to high atom economy. Yet, developing an efficient, one-pot biocatalytic cascade remains challenging. Combined with the selectivity of biological enzymes and tunable activity of nanozymes, we herein demonstrate an effective bio-nanozyme cascade formed by glucose oxidase (GOx) and in situ-generated nanoceria. The prepared H₂O₂-nanoceria complex shows strong oxidative activity for common chromogenic substrates under physiological conditions, which are the optimal reaction conditions for most biological enzymes. Interestingly, GOx not only provides H₂O₂ for the second step reaction but also simultaneously leads to 7.4-fold enhancement of activity. We characterized the process of in situ generation of nanoceria at pH 7.0 and how proteins boost the activity by enhancing product desorption. In addition, the proposed one-pot bio-nanozyme cascade shows high stability and analytical performance for serum glucose with a detection limit of 5 μM.

Applications of electron spin resonance spectroscopy in photoinduced nanomaterial charge separation and reactive oxygen species generation

Nano-metals, nano-metal oxides, and carbon-based nanomaterials exhibit superior solar-to-chemical/photo-electron transfer properties and are potential candidates for environmental remediations and energy transfer. Recent research effort focuses on enhancing the efficiency of photoinduced electron-hole separation to improve energy transfer in catalytic reactions. Electron spin resonance (ESR) spectroscopy has been used to monitor the generation of electron/hole and reactive oxygen species (ROS) during nanomaterial-mediated photocatalysis. Using ESR coupled with spin trapping and spin labeling techniques, the underlying photocatalytic mechanism involved in the nanomaterial-mediated photocatalysis was investigated. In this review, we briefly introduced ESR principle and summarized recent advancements using ESR spectroscopy to characterize electron-hole separation and ROS production by different types of nanomaterials.

Layer-by-Layer Electrodeposition of FTO/TiO2 /Cux O/CeO2 (1 < x < 2) Photocatalysts with High Peroxidase-Like Activity by Greatly Enhanced Singlet Oxygen Generation

Inorganic nanomaterials have attracted much attention as enzyme mimics because of simple and stable spatial conformation of those artificially synthesized nanocatalysts. Cu₂ O, as an important kind of narrow band gap semiconductor, is identified as effective as visible-light-driven photocatalysts, which can catalyze decomposition of H₂ O₂ into reactive oxygen species. Moreover, after forming Cu_x O/CeO₂ hybrids, the strongly coupled interface between the two components will further improve their catalytic performance. In this paper, the authors try to construct FTO/TiO₂ /Cu_x O/CeO₂ (1 < x < 2) nanohybrids with such a kind of active interface via a layer-by-layer electrodeposition strategy by aid of the following surface etching process. It is found that FTO/TiO₂ /Cu_x O/CeO₂ exhibits good peroxidase mimic activity in the dark but much better performance under visible light irradiation (λ ≥ 420 nm) during catalytic oxidation of 3,3',5,5'-tetramethylbenzidine substrates in the presence of H₂ O₂ . Detailed characterizations disclose that the construction of TiO₂ /Cu₂ O pn-heterojunctions do effectively accelerate separation of photogenerated carriers, and the formation of a highly active Cu_x O/CeO₂ interface is synergistically favorable for selectively generating singlet oxygen to boost the catalytic performance of FTO/TiO₂ /Cu_x O/CeO₂ .

Nucleoside-based fluorescent carbon dots for discrimination of metal ions

Carbon dots (Cdots) play an important role in many biological and chemical applications. To prepare strongly fluorescent Cdots, the starting material should contain nitrogen in addition to carbon. Nucleobases are nitrogen rich with interesting metal binding properties. In this work, we prepared a series of Cdots with citrate as the carbon source, and ethylenediamine, adenosine, cytidine, thymidine or guanosine as the respective nitrogen sources. The resulting Cdots were all fluorescent with the ethylenediamine sample being the most strongly emissive. These Cdots were then tested for their metal sensitivity and all tested metal ions can quench their fluorescence. The fluorescence of the G-Cdots prepared with guanosine was quenched most efficiently by Cu2+, while the Cdots prepared with ethylenediamine were more sensitive to Hg2+. With the differential quenching by different metal ions, we prepared a sensor array to discriminate multiple metal ions, and quantified Cu2+ and Hg2+ at the same time. Our work has expanded the range of starting materials for preparing Cdots and showed that by tuning the precursor composition, Cdots with different optical and metal binding properties can be obtained, which is useful in constructing a sensing platform for a large number of metal ions.

Light-Driven Cascade Mitochondria-to-Nucleus Photosensitization in Cancer Cell Ablation

Nuclei and mitochondria are the only cellular organelles containing genes, which are specific targets for efficient cancer therapy. So far, several photosensitizers have been reported for mitochondria targeting, and another few have been reported for nuclei targeting. However, none have been reported for photosensitization in both mitochondria and nucleus, especially in cascade mode, which can significantly reduce the photosensitizers needed for maximal treatment effect. Herein, a light-driven, mitochondria-to-nucleus cascade dual organelle cancer cell ablation strategy is reported. A functionalized iridium complex, named BT-Ir, is designed as a photosensitizer, which targets mitochondria first for photosensitization and subsequently is translocated to a cell nucleus for continuous photodynamic cancer cell ablation. This strategy opens new opportunities for efficient photodynamic therapy.

Europium coordination polymer particles based electrospun nanofibrous film for point-of-care testing of copper (II) ions

Excess free copper in serum has been identified to induce neurodegenerative diseases such as Alzheimer's disease, thus it is very important to determine copper (II) ions (Cu²⁺) content for human health test. Herein we developed a point-of-care testing (POCT) platform through a luminescence "on-off" recognition mechanism of serum copper. Microsized europium coordination polymer particles (Eu-CPs), which was prepared with citric acid (CA) and europium nitrate hexahydrate through a hydrothermal route, were then successfully loaded with the mixture of 2,6-pyridinedicarboxylic acid (DPA) and poly(vinyl alcohol) (PVA) to form electrospun nanofibrous films (ENFFs). The as-prepared Eu-CPs/DPA/PVA ENFFs exhibited red emission at 618 nm when exciting at 280 nm, with the quantum yields of 22.2% owing to the antenna effect from DPA to Eu³⁺. Furthermore, the strong luminescence could be selectively quenched by Cu²⁺ through coordination with DPA to interrupt the antenna effect. With that, Cu²⁺ was successfully detected in the range of 2-45 μM with a detection of limit of 1.3 μM, well matching with the requirement of clinic test of excess free copper induced neurodegenerative diseases. As a proof of concept at last, this POCT platform was used to detect free copper in spiked serum samples with a recovery of 101.1%-105.2%, demonstrating that this platform provides significant potential for use in clinical test.

Nanozyme's catching up: activity, specificity, reaction conditions and reaction types

Nanozymes aim to mimic enzyme activities. In addition to catalytic activity, nanozymes also need to have specificity and catalyze biologically relevant reactions under physiological conditions to fit in the definition of enzyme and to set nanozymes apart from typical inorganic catalysts. Previous discussions in the nanozyme field mainly focused on the types of reactions or certain analytical, biomedical or environmental applications. In this article, we discuss efforts made to mimic enzymes. First, the catalytic cycles are compared, where a key difference is specific substrate binding by enzymes versus non-specific substrate adsorption by nanozymes. We then reviewed efforts to engineer and surface-modify nanomaterials to accelerate reaction rates, strategies to graft affinity ligands and molecularly imprinted polymers to achieve specific catalysis, and methods to bring nanozyme reactions to neutral pH and ambient temperature. Most of the current nanozyme reactions used a few model chromogenic substrates of no biological relevance. Therefore, we also reviewed efforts to catalyze the conversion of biomolecules and biopolymers using nanozymes. By the efforts to close the gaps between nanozymes and enzymes, we believe nanozymes are catching up rapidly. Still, challenges exist in materials design to further improve nanozymes as true enzyme mimics and achieve impactful applications.

Oxygen doping through oxidation causes the main active substance in g-C3N4 photocatalysis to change from holes to singlet oxygen

Improving the photocatalytic activity of graphitic carbon nitride (g-C₃N₄) for organic pollutant removal from water and in-depth study of the effect of oxygen doping on its photocatalytic mechanism are deserving of research attention. Oxygen-doped g-C₃N₄ with a suitable degree of oxidation was prepared by oxidation using concentrated acid-ultrasound double oxidation. Oxygen doping by oxidation changed the physical and chemical properties of g-C₃N₄, and improved its rhodamine B photocatalytic degradation efficiency. The physical and chemical properties of g-C₃N₄ were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and zeta potential analysis, among other methods. The photocatalytic mechanism was also studied in depth using photoluminescence, electrochemical impedance spectroscopy, transient photocurrent measurements, diffuse reflectance spectroscopy, and Mott-Schottky plots. An appropriate degree of oxidation introduced pit-like defects rich in oxygen-containing functional groups onto the g-C₃N₄ surface, which improves its photocatalytic performance. Furthermore, in the photocatalytic process of oxidized g-C₃N₄, oxygen doping was confirmed to change the main active substance of g-C₃N₄ photocatalysis from holes to singlet oxygen. This study provides an in-depth explanation of the photocatalytic mechanism of g-C₃N₄ doped with oxygen, which provides guidance and reference for the design of photocatalysts for organic pollutant removal from water and the analysis of photocatalytic mechanisms for water treatment.

Expanding the Limits of Photodynamic Therapy: The Design of Organelles and Hypoxia-Targeting Nanomaterials for Enhanced Photokilling of Cancer

Photodynamic therapy (PDT) is a minimally invasive clinical protocol that combines a nontoxic photosensitizer (PS), appropriate visible light, and molecular oxygen for cancer treatment. This triad generates reactive oxygen species (ROS) in situ, leading to different cell death pathways and limiting the arrival of nutrients by irreversible destruction of the tumor vascular system. Despite the number of formulations and applications available, the advancement of therapy is hindered by some characteristics such as the hypoxic condition of solid tumors and the limited energy density (light fluence) that reaches the target. As a result, the use of PDT as a definitive monotherapy for cancer is generally restricted to pretumor lesions or neoplastic tissue of approximately 1 cm in size. To expand this limitation, researchers have synthesized functional nanoparticles (NPs) capable of carrying classical photosensitizers with self-supplying oxygen as well as targeting specific organelles such as mitochondria and lysosomes. This has improved outcomes in vitro and in vivo. This review highlights the basis of PDT, many of the most commonly used strategies of functionalization of smart NPs, and their potential to break the current limits of the classical protocol of PDT against cancer. The application and future perspectives of the multifunctional nanoparticles in PDT are also discussed in some detail.

Ultrastable AgBiS2 Hollow Nanospheres with Cancer Cell-Specific Cytotoxicity for Multimodal Tumor Therapy

Specific cytotoxicity for catalytic nanomedicine triggered by the tumor microenvironment (TME) has attracted increasing interest. In this work, we prepared AgBiS₂ hollow nanospheres with narrow bandgaps via rapid precipitation in a weakly polar solvent, which lowered the intrinsic energy gap for the active production of highly reactive hydroxyl radicals (^•OH), especially in the TME. The as-prepared AgBiS₂ hollow nanospheres exhibited enhanced optical absorption and high photothermal conversion efficiency (44.2%). In addition, the hollow structured AgBiS₂ nanospheres were found to have a peroxidase-mimicking feature to induce cancer cell-specific cytotoxicity while exhibiting negligible cytotoxicity toward normal cells, which might be attributed to the efficient production of highly reactive ^•OH originating from the overexpression H₂O₂ in the TME caused by surface catalysis. In particular, the cancer cell-specific cytotoxicity of the nanospheres was greatly enhanced both in vitro and in vivo upon irradiation with a near-infrared (NIR) laser (808 nm). The above-mentioned features of the hollow structured AgBiS₂ will make it a promising candidate for tumor therapy.

Designing signal-on sensors by regulating nanozyme activity

Nanozymes are nanomaterials with enzyme-like activities. Compared to natural enzymes, nanozymes are more stable and cost-effective, and they have unique properties due to their nanoscale size and surface chemistry. In this review, we summarize 'signal-on' nanozyme-based sensors for detecting metal ions, anions, small molecules and proteins. Since protein-based enzymes are already highly active, they were used to detect their inhibitors, resulting in 'signal-off' sensors. On the other hand, for nanozymes, target molecules were detected either as a promotor of nanozyme activity or for its ability to selectively remove nanozyme inhibitors. In both cases, 'signal-on' detection was achieved. We classify the commonly used nanozymes based on their composition such as metal oxide, gold nanoparticles and other nanomaterials, most of which belong to the oxidase, peroxidase and catalase mimics. The nanozymes can catalyze the oxidation of colorless or non-fluorescent substrates to produce a visual or fluorescent signal. Based on this, this article presents some typical 'turn-on' and 'turn-off-on' sensors, and we critically review their design principles. At the end, further perspectives for the nanozyme-based sensors are outlined.

Orthogonal Adsorption of Carbon Dots and DNA on Nanoceria

Carbon dots (CDs) are highly fluorescent nanomaterials with surface carboxyl and amino groups. However, their exact structure remains under debate. In this work, we probed the surface properties of CDs by physically adsorbing them onto various nanomaterials. Three types of nanomaterials, including CeO₂ nanoparticles (nanoceria), gold nanoparticles, and graphene oxide were tested. Among them, nanoceria strongly adsorbed the CDs and quenched their fluorescence. For the tested anions to compete with the CDs for adsorption, only phosphate and F^- induced desorption of the CDs from nanoceria, and the phosphate-induced desorption was less compared to that by F^-. This was opposite to the desorption of DNA from nanoceria, where phosphate induced more DNA desorption. Furthermore, using calcein and fluorescein as representative dyes for comparison, we conclude that the CDs might use their carboxyl groups to adsorb on nanoceria, while DNA uses its phosphate backbone for adsorption. This difference may explain their occupying different surface sites on nanoceria and different displacement by phosphate and F^-. Using nanomaterials as probes to understand the surface properties of CDs is effective, and such understanding might in turn be used for building hybrid materials for applications.

Light-activated nanozymes: catalytic mechanisms and applications

Recently, nanozymes have attracted enormous interest for their high stability, low cost and various enzyme-like activities. In nature, many biochemical reactions require light. Recently, introducing light to nanozymes has also been reported, especially for photosensitized oxygen activation. Compared to normal nanozymes, light-activated nanozymes possess several advantages including light-regulated activity, using molecular oxygen as a green oxidant, and often higher activity can be achieved. Herein, we summarize light-activated nanozymes, starting from their photophysical processes and identification of reactive oxygen species (ROS). Although the types of light-activated nanozymes are still quite limited and cannot yet mimic the same reactions as natural photo-related enzymes, they have widened the range of nanozymes. A few specific applications are highlighted, including sensing, chemical synthesis, degradation of organic pollutants, and cleavage and repair of DNA. Finally, a few future research opportunities are discussed.