Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes

Enhanced hydrogen production by robust covalent biohybrid based on cell membrane specific click chemistry

The light-driven semiconductor-bacteria hybrid holds great potential for synthesis of diverse solar chemicals and fuels. However, the efficiency of electron transfer between biotic-abiotic interface often suffers from the insufficient robustness of biohybrid, which significantly imped its performance and applications. Here, a highly robust biohybrid was established by specifically and covalently grafting carbon quantum dots (CDs) onto bacterial cell membrane via the copper-catalyzed azide-Alkyne click (CuAAC) reaction. The formation of covalent bonds dramatically enhanced the robustness of hybrid and further shorten the distance of biotic-abiotic interface, endowing long-term stability under different conditions. Consequently, the cell membrane-specific covalent-biohybrid exhibited a 7.3-fold higher hydrogen production due to enhanced system stability, higher load of CDs on the cell surface and direct electron transfer efficiency. This work demonstrated a new and promising approach to improve the robustness and performance of photocatalytic semiconductor-bacteria hybrid system, which would further diversify the toolbox for solar chemicals/fuels production.

Smartphone-driven handheld monitoring of Cr(VI) and living cell imaging using nitrogen-doped yellow fluorescence carbon dots

User-friendly, field-portable, and direct read-out mini-device is indispensable for under-developed countries or resources-insufficient areas. Herein, a field-portable handheld monitoring platform incorporating smartphone, 3D-printed device, and test strip stained with yellow fluorescence (FL) carbon dots (YCDs) proves the efficacy for real-time quantitation of Cr(VI) by a consecutive FL color transformation. YCDs were synthesized with starch, rhodamine 6G, and L-lysine through hydrothermal procedure, manifesting intriguing yellow-emitting behavior locating at 556 nm under 470 nm excitation. With increasing Cr(VI) concentration, FL intensity of YCDs at 556 nm is significantly quenched, accompanying by a continuous FL colour alteration from bright yellow to dark yellow. Inspired by above FL phenomena, YCDs-based test strips are fabricated to realize point-of-care quantifying of Cr(VI). Inspiringly, developed field-portable handheld monitoring platform is further generalized to on-site quantify Cr(VI) concentration in tap water based on wide linear ranges of 0-800 μM, and low limit of detection of 0.349 μM. Furthermore, laser confocal imaging of HeLa cells corroborates that YCDs can be utilized for visual determination of Cr(VI) in living cells, illustrating that YCDs presents powerful biosensing application prospect.

Insights into the Direct Photoelectron Transfer Mechanism in Cofactor-free Redox Carbon Dots and Cytochrome c Nitrite Reductase Biohybrids Responsible for Ammonia Synthesis

Highly efficient NH3 production has been reported to be achieved by photosensitizing Shewanella oneidensis MR-1 using carbon dots (CDs). During this process, cytochrome c nitrite reductase (NrfA) is regarded as the rate-limiting enzyme. However, the precise electron transfer mechanism between CDs and NrfA remains unclear. Herein, a hybrid photosynthetic system composed of NrfA and redox CDs (NrfA/R-CDs) was constructed, achieving a maximum NH3 production rate of 12.5 ± 1.1 μmol (NH3)·mg-1 (NrfA)·h-1. R-CDs with aromatic ketone groups could store photoinduced electrons, enhancing carrier separation efficiency. These stored photoelectrons were capable of being directly transferred to NrfA without the need of cofactors. Even under dark conditions, direct electron transfer occurred from the stored photoelectrons in R-CDs to NrfA, providing an indirect illumination approach to reduce phototoxicity to NrfA. Molecular docking and dynamics simulations demonstrated the formation of a stable complex between the heme 2 region of NrfA and R-CDs. The short distance (10 Å) and π-electronic interactions between R-CDs and heme 2 facilitated the direct electronic transfer process. This study provides a comprehensive understanding of the interfacial photoelectron transfer mechanism and guidelines for constructing nanomaterials with photoelectron storage and release properties.

Spatially resolved charge-transfer kinetics at the quantum dot-microbe interface using fluorescence lifetime imaging microscopy

Integrating the optoelectronic properties of quantum dots (QDs) with biological enzymatic systems to form microbe-semiconductor biohybrids offers promising prospects for both solar-to-chemical conversion and light-modulated biochemical processes. Developing these nano-bio hybrid systems necessitates a deep understanding of charge-transfer dynamics at the nano-bio interface. Photoexcited carrier transfer from QDs to microbes is driven by complex interactions, with emerging insights into the relevant thermodynamic and kinetic factors. The heterogeneities of both microbes and QD ensembles pose significant challenges in mechanistic understanding, which is critical for designing advanced nano-bio hybrids. We used fluorescence lifetime imaging microscopy to analyze charge transfer between a CdSe QD film and Shewanella oneidensis microbes. We correlated the spatiotemporal fluorescence data with an analytical model. Our analysis revealed two distinct distributions of QD de-excitation pathways. The characteristics of these distributions: 1) a faster transfer rate ([Formula: see text]), with a lower acceptor number ([Formula: see text]) and 2) a slower transfer rate ([Formula: see text]) with a higher acceptor number ([Formula: see text]). We assign these distributions to the indirect and direct electron transfer mechanisms, respectively. Our findings demonstrate how spectroscopic imaging can uncover fundamental electron transfer mechanisms at complex interfaces, offering valuable design principles for future nano-bio hybrids.

Unveiling the Photoluminescence Mechanism of Carbonized Polymer Dots: Evolution and Synergistic Photoluminescence of Multiple Molecular Fluorophores

Carbonized polymer dots (CPDs) are a class of exceptional fluorescent materials with diverse applications. However, their photoluminescence (PL) mechanism remained enigmatic and controversial, hindering further development and application. While molecular fluorophores explain primary fluorescence in some CPDs, the overall PL of CPDs still cannot be fully explained, such as their excitation-dependent behaviors, let alone the modulation of PL. Besides, the extracted molecular fluorophores are dissociative, raising questions including the purity of CPDs and whether the PL of CPDs nanoparticles come from these molecular fluorophores. Here, the emission around 510 nm of citric acid-ethylenediamine CPDs (CA-EDA CPDs) was proved to be contributed by a molecular fluorophore, which evolved from another molecular fluorophore following the quantum confinement effect. Further research revealed the whole PL mechanism of CA-EDA CPDs to be the evolution and synergistic PL of multiple molecular fluorophores linked on CPDs nanoparticle. The evolution of molecular fluorophores had also been observed in another two systems, indicating the universality of this mechanism, and a new approach for regulating the optical properties of CPDs was put forward inspired by this mechanism. This study not only refined the PL mechanism but also paved the way for future advancements of CPDs.

Turning the band alignment of carbon dots for visible-light-driven enzymatic asymmetric reduction of aromatic ketone

Keto reductases are crucial NAD(P)H-dependent enzymes used for the enantioselective synthesis of alcohols from prochiral ketones. Typically, the NADPH cofactor is regenerated through a second enzyme and/or substrate. However, photocatalytic cofactor regeneration using water as a sacrificial electron and hydrogen donor presents a promising alternative, albeit a challenging one. Herein we fabricated several nitrogen-doped carbon dots (CDs) with visible light absorption properties, good water solubility and biocompatibility for photocatalytic regeneration of NADPH. The CD with a smaller size and suitable redox potential gave the highest NADPH yield (55.7 %). Based on this, NADPH-dependent aldo-keto reductase crosslinked aggregates (AKR-CLEs) were initially applied as a stable biocatalyst to reduce the prochiral ketone. (S)-1-(2-Chlorophenyl) ethanol, an intermediate for LPA1R antagonists, was obtained in 65.3 % yield and 99.99 % enantiomeric excess (ee) under visible light irradiation. The isotope tracer experiment confirmed that water is the hydrogen donor in this light-driven, photo-enzymatic asymmetric hydrogenation system. This method is useful for the sustainable synthesis of chiral alcohols. Moreover, the general principle of utilizing water as the sacrificial hydrogen and electron donor holds potential for application in other redox cofactor regeneration systems.

Photoinduced electron transfer enables cytochrome P450 enzyme-catalyzed reaction cycling

Cytochrome P450 enzymes (CYPs), the members of the largest superfamily of enzymes in plant kingdom, catalyze a variety of functional group transformations involved in metabolite biosynthesis, end-product derivatization, and exogeneous molecule detoxification. Nevertheless, CYPs' functional characterization and practically industrial application have been largely encumbered by their critical dependency on the reducing equivalent for the catalytic cycling, driven by the tedious electron relay mediated by CYP reductase (CPR). Here, we report a photoinduced electron transfer system that initiates and sustains the CYP-catalyzed reaction cycling. Using Camptotheca acuminata CYP72A565-catalyzed carbon-carbon cleavage reaction, a key biosynthetic reaction in the biosynthesis of plant-derived antitumor monoterpene indole alkaloid camptothecin, as a representative CYP-catalyzed reaction model, we identified eosin Y (EY) and triethanolamine (TEOA) as an efficient photosensitizer/sacrificial reagent pair for the photoinduced electron generating system. The C. acuminata camptothecin 10-hydroxylase-catalyzed regioselective C10-hydroxylation of camptothecin into 10-hydroxycamptothecin could be enabled by the photoinduced electron transfer system, demonstrating that the EY/TEOA pair serves as an efficient surrogate for membranous CPR and can be expanded to other CYP-catalyzed reaction cycling. The catalytic efficiency of the photoinduced electron transfer-driven CYP-catalyzed cycling exceeds that of the native NADPH-dependent CPR-supported CYP-catalyzed reaction, thereby circumventing the dependency on both NADPH and the reductase CPR. The present study provides a photoinduced electron generating and transferring system as an efficient and facile alternative to membranous NADPH-dependent CPR, offering a new avenue for CYP-mediated conversion of complex bioactive natural products using synthetic biology approaches.

Organic semiconductor bulk heterojunctions for solar-to-chemical conversion: recent advances and challenges

Solar fuel production involving the conversion of solar energy directly into chemical fuels such as hydrogen and valuable chemicals using photoelectrochemical (PEC) cells and photocatalysts (PCs) offers a promising avenue for sustainable energy while reducing carbon emissions. However, existing PEC cells and PCs fall short of economic viability due to their low solar-to-chemical (STC) conversion efficiency associated with the employed semiconductors, highlighting the clear need for identifying ideal semiconductor materials. Organic semiconductors (OSs), π-conjugated carbon-based materials, have emerged as promising candidates for enhancing STC conversion efficiency due to their remarkable optoelectrical properties, which can be readily adjustable through molecular engineering. In particular, the use of OS bulk heterojunctions (BHJs) consisting of intermixed electron-donating and electron-accepting OSs facilitates efficient charge generation under illumination, thereby contributing to enhanced STC conversion efficiency. This review explores the recent advancements in the rational design of OS materials and approaches aimed at enhancing the performance of BHJ-based PEC cells and PCs for solar-driven production of hydrogen and valuable chemicals. The discussion also introduces new perspectives to address the remaining challenges in this field.

High-performance ratiometric fluorescence detection and removal of tetracycline in milk based on CDs@ZSM-5:Eu3

Exploring materials with the dual functionality of detecting and removing tetracycline (TC) residues is crucial because of the environmental and health risks posed by antibiotic overuse. This study introduces a dual-emissive luminescent probe, CDs@ZSM-5:Eu3+, created through a solvent-free method combined with subsequent Eu3+ion exchange. The nanocomposite's blue emission, originating from carbon dots (CDs), is quenched by TC via an internal filtering effect, while an antenna effect triggers a strong red fluorescence of a TC-Eu3+chelate. The ratiometric fluorescence changes in CDs@ZSM-5:Eu3+ endow a self-calibrated sensing mechanism for TC, offering a low detection limit of 5.04 nM and a broad detection range of 0.01-50 μM. Demonstrated in real milk samples, the probe exhibits high selectivity and accuracy in detecting TC. The nanocomposite also displayed an impressive TC removal capacity of 238.1 mg g-1 in water, ascribing to the enrichment and electrostatic attraction effects of ZSM-5 toward TC molecules. This research offers a facile strategy for constructing multifunctional zeolite-based hybrids for simultaneous TC detection and removal from aqueous solutions.

Modulating carbon dots from aggregation-caused quenching to aggregation-induced emission and applying them in sensing, imaging and anti-counterfeiting

Aggregation Induced Emission Carbon Dots (AIE-CDs) address the problem of conventional CDs being quenched in the solid-state. However, there are still challenges in comprehending the luminescence mechanism. This work proposed a strategy for preparing green, yellow, and near-infrared CDs by modifying the functional groups on the precursor from hydroxyl and amino to p-methylenediamine, in which electronic supply capacity determined the redshift. Additionally, The CDs' properties transformed from Aggregation-Caused Quenching (ACQ) to AIE was realized by substituting non-rotatable hydroxyl or amino groups with the rotatable p-methylenediamine on the precursor. The resulting CDs were then applied in multifield. C-CDs was used for ratiometric detection of Al3+ and F- in pure water through three methods including fluorometer, test strip and smartphone. R-CDs was used for imaging cell nucleus and zebrafish. NIR-CDs (λem = 676 nm) exhibits dual emission, AIE and phosphorescent characteristics was used for triple anti-counterfeiting and binary information encryption. In summary, our finding presented a strategy for preparing multicolor CDs, proposed a mechanism for the transition of CDs from ACQ to AIE, and explore their multiple applications in anti-counterfeiting, information encapsulation, sensing and imaging.

Structure of a biohybrid photosystem I-platinum nanoparticle solar fuel catalyst

Biohybrid solar fuel catalysts leverage natural light-driven enzymes to produce valuable fuel products. One useful biological platform for such a system is photosystem I, a pigment-protein complex that captures sunlight and converts it into chemical energy with near unity quantum efficiency, which generates low potential reducing equivalents for metabolism. Realizing and understanding the molecular basis for an approach that utilizes those electrons and stores solar energy as a fuel is therefore appealing. Here, we report the 2.27-Å global resolution cryo-EM structure of a photosystem I complex with bound platinum nanoparticles that catalyzes light-driven H2 production. The platinum nanoparticle binding sites and possible stabilizing interactions are described. Overall, the investigation reveals a direct structural look at a photon-to-fuels photosynthetic biohybrid system.

Enhanced Interfacial Electron Transfer in Photocatalyst-Natural Enzyme Coupled Artificial Photosynthesis System: Tuning Strategies and Molecular Simulations

Laccase is capable of catalyzing a vast array of reactions, but its low redox potential limits its potential applications. The use of photocatalytic materials offers a solution to this problem by converting absorbed visible light into electrons to facilitate enzyme catalysis. Herein, MIL-53(Fe) and NH2-MIL-53(Fe) serve as both light absorbers and enzyme immobilization carriers, and laccase is employed for solar-driven chemical conversion. Electron spin resonance spectroscopy results confirm that visible light irradiation causes rapid transfer of photogenerated electrons from MOF excitation to T1 Cu(II) of laccase, significantly increasing the degradation rate constant of tetracycline (TC) from 0.0062 to 0.0127 min-1. Conversely, there is only minimal or no electron transfer between MOF and laccase in the physical mixture state. Theoretical calculations demonstrate that the immobilization of laccase's active site and its covalent binding to the metal-organic framework surface augment the coupled system's activity, reducing the active site accessible from 27.8 to 18.1 Å. The constructed photo-enzyme coupled system successfully combines enzyme catalysis' selectivity with photocatalysis's high reactivity, providing a promising solution for solar energy use.

Photosynthetic biohybrid systems for solar fuels catalysis

Photosynthetic reaction center (RC) proteins are finely tuned molecular systems optimized for solar energy conversion. RCs effectively capture and convert sunlight with near unity quantum efficiency utilizing light-induced directional electron transfer through a series of molecular cofactors embedded within the protein core to generate a long-lived charge separated state with a useable electrochemical potential. Of current interest are new strategies that couple RC chemistry to the direct synthesis of energy-rich compounds. This Feature Article highlights recent work from our lab on RC and RC-inspired hybrid systems that capture the Sun's energy and convert it to chemical energy in the form of H2, a carbon-neutral energy source derived from water. Biohybrids made from the Photosystem I (PSI) RC are among the best photocatalytic H2-producing protein hybrids to date. Targeted self-assembly strategies that couple abiotic catalysts to PSI translate to catalyst incorporation at intrinsic PSI sites within thylakoid membranes to achieve complete solar water-splitting systems. RC-inspired biohybrids interface synthetic photosensitizers and molecular catalysts with small proteins to create photocatalytic systems and enable the spectroscopic discernment of the structural features and electron transfer processes that underpin solar-driven proton reduction. In total, these studies showcase the incredible scientific opportunities photosynthetic biohybrid research provides for harnessing the optimal qualities of both artificial and natural photosynthetic systems and developing materials that capture, convert, and store solar energy as a fuel.

Influence of Electron Donors on the Charge Transfer Dynamics of Carbon Nanodots in Photocatalytic Systems

Carbon nanodots (CNDs) are nanosized light-harvesters emerging as next-generation photosensitizers in photocatalytic reactions. Despite their ever-increasing potential applications, the intricacies underlying their photoexcited charge carrier dynamics are yet to be elucidated. In this study, nitrogen-doped graphitic CNDs (NgCNDs) are selectively excited in the presence of methyl viologen (MV2+, redox mediator) and different electron donors (EDs), namely ascorbic acid (AA) and ethylenediaminetetraacetic acid (EDTA). The consequent formation of the methyl viologen radical cation (MV•+) is investigated, and the excited charge carrier dynamics of the photocatalytic system are understood on a 0.1 ps-1 ms time range, providing spectroscopic evidence of oxidative or reductive quenching mechanisms experienced by optically excited NgCNDs (NgCNDs*) depending on the ED implemented. In the presence of AA, NgCNDs* undergo oxidative quenching by MV2+ to form MV•+, which is short-lived due to dehydroascorbic acid, a product of photoinduced hole quenching of oxidized NgCNDs. The EDTA-mediated reductive quenching of NgCNDs* is observed to be at least 2 orders of magnitude slower due to screening by EDTA-MV2+ complexes, but the MV•+ population is stable due to the irreversibly oxidized EDTA preventing a back reaction. In general, our methodology provides a distinct solution with which to study charge transfer dynamics in photocatalytic systems on an extended time range spanning 10 orders of magnitude. This approach generates a mechanistic understanding to select and develop suitable EDs to promote photocatalytic reactions.

Engineering of bespoke photosensitiser-microbe interfaces for enhanced semi-artificial photosynthesis

Biohybrid systems for solar fuel production integrate artificial light-harvesting materials with biological catalysts such as microbes. In this perspective, we discuss the rational design of the abiotic-biotic interface in biohybrid systems by reviewing microbes and synthetic light-harvesting materials, as well as presenting various approaches to coupling these two components together. To maximise performance and scalability of such semi-artificial systems, we emphasise that the interfacial design requires consideration of two important aspects: attachment and electron transfer. It is our perspective that rational design of this photosensitiser-microbe interface is required for scalable solar fuel production. The design and assembly of a biohybrid with a well-defined electron transfer pathway allows mechanistic characterisation and optimisation for maximum efficiency. Introduction of additional catalysts to the system can close the redox cycle, omitting the need for sacrificial electron donors. Studies that electronically couple light-harvesters to well-defined biological entities, such as emerging photosensitiser-enzyme hybrids, provide valuable knowledge for the strategic design of whole-cell biohybrids. Exploring the interactions between light-harvesters and redox proteins can guide coupling strategies when translated into larger, more complex microbial systems.

The role of reduced graphene oxide on mitigation of lead phytotoxicity in Triticum aestivum L.plants at morphological and physiological levels

Rapid global industrialization and an increase in population have enhanced the risk of heavy metals accumulation in plant bodies to disrupt the morphological, biochemical, and physiological processes of plants. To cope with this situation, reduced graphene oxide (rGO) NPs were used first time to mitigate abiotic stresses caused in plant. In this study, rGO NPs were synthesized and reduced with Tecoma stans plant leave extract through modified Hummer's methods. The well prepared rGO NPs were characterized by ultra-violet visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Zeta potential, and scanning electron microscopy (SEM). However, pot experiment was conducted with four different concentrations (15, 30, 60, 120 mg/L) of rGO NPs and three different concentrations (300, 500,700 mg/L) of lead (Pb) stress were applied. To observe the mitigative effects of rGO NPs, 30 mg/L of rGO NPs and 700 mg/L of Pb were used in combination. Changes in morphological and biochemical characteristics of wheat plants were observed for both Pb stress and rGO NPs treatments. Pb was found to inhibit the morphological and biochemical characteristics of plants. rGO NPs alone as well as in combination with Pb was found to increase the chlorophyll content of wheat plants. Under Pb stress conditions and rGO NPs treatments, antioxidant enzyme activities like ascorbate peroxidases (APX), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were observed. Current findings revealed that greenly reduced graphene oxide NPs can effectively promote growth in wheat plants under Pb stress by elevating chlorophyll content of leaves, reducing the Pb uptake, and suppressing ROS produced due to Pb toxicity.

Delineating the core and surface state heterogeneity of carbon dots during electron transfer

Exploring the heterogeneity of carbon dots (C-Dots) is challenging because of the existence of complex structural diversity, and it is a demanding task for the development and designing of efficient C-Dots for various applications. Herein, we studied the role of the core state and surface state of C-Dots in heterogeneity via the successful investigation of the electron transfer (ET) process between different (blue, green, and red) emitting C-Dots and an electron acceptor methyl viologen (MV2+) using steady-state and time-resolved fluorescence and ultrafast transient absorption (TA) spectroscopic techniques. Selective excitation in the steady-state and time-resolved mode shows that the ET ability of the core state is higher than that of the surface state. Moreover, the kinetics of MV+˙ generation was probed using TA spectroscopy after the excitation of the core and surface state, where we observed that the surface state becomes less efficient due to the presence of an oxygen-containing functional group in the surface state, which acts as an electron scavenger. Moreover, the heterogeneity of the core and surface state was explored through the detection of the MV+˙ generation yield after the irradiation of UV and visible light (exciting the core and surface state). The result indicates that the graphitic nitrogen content in the core state and the oxygen-containing functional group in the surface state play an important role in the heterogeneity in the structure and the ET process. Our findings on the fundamental understanding of the heterogeneity of different emissive C-Dots will provide a new way of designing and developing a metal-free light-harvesting system.

Electrostatic [FeFe]-hydrogenase-carbon nitride assemblies for efficient solar hydrogen production

The assembly of semiconductors as light absorbers and enzymes as redox catalysts offers a promising approach for sustainable chemical synthesis driven by light. However, achieving the rational design of such semi-artificial systems requires a comprehensive understanding of the abiotic-biotic interface, which poses significant challenges. In this study, we demonstrate an electrostatic interaction strategy to interface negatively charged cyanamide modified graphitic carbon nitride (NCNCNX) with an [FeFe]-hydrogenase possessing a positive surface charge around the distal FeS cluster responsible for electron uptake into the enzyme. The strong electrostatic attraction enables efficient solar hydrogen (H2) production via direct interfacial electron transfer (DET), achieving a turnover frequency (TOF) of 18 669 h-1 (4 h) and a turnover number (TON) of 198 125 (24 h). Interfacial characterizations, including quartz crystal microbalance (QCM), photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photovoltage spectroscopy (IMVS), and transient photocurrent spectroscopy (TPC) have been conducted on the semi-artificial carbon nitride-enzyme system to provide a comprehensive understanding for the future development of photocatalytic hybrid assemblies.

Light-driven Transformation of Carbon Monoxide into Hydrocarbons using CdS@ZnS : VFe Protein Biohybrids

Enzymatic Fisher-Tropsch (FT) process catalyzed by vanadium (V)-nitrogenase can convert carbon monoxide (CO) to longer-chain hydrocarbons (>C2) under ambient conditions, although this process requires high-cost reducing agent(s) and/or the ATP-dependent reductase as electron and energy sources. Using visible light-activated CdS@ZnS (CZS) core-shell quantum dots (QDs) as alternative reducing equivalent for the catalytic component (VFe protein) of V-nitrogenase, we first report a CZS : VFe biohybrid system that enables effective photo-enzymatic C-C coupling reactions, hydrogenating CO into hydrocarbon fuels (up to C4) that can be hardly achieved with conventional inorganic photocatalysts. Surface ligand engineering optimizes molecular and opto-electronic coupling between QDs and the VFe protein, realizing high efficiency (internal quantum yield >56 %), ATP-independent, photon-to-fuel production, achieving an electron turnover number of >900, that is 72 % compared to the natural ATP-coupled transformation of CO into hydrocarbons by V-nitrogenase. The selectivity of products can be controlled by irradiation conditions, with higher photon flux favoring (longer-chain) hydrocarbon generation. The CZS : VFe biohybrids not only can find applications in industrial CO removal for high-value-added chemical production by using the cheap, renewable solar energy, but also will inspire related research interests in understanding the molecular and electronic processes in photo-biocatalytic systems.

Magnetic Polaron States in Photoluminescent Carbon Dots Enable Hydrogen Peroxide Photoproduction

Photoactivation of aspartic acid-based carbon dots (Asp-CDs) induces the generation of spin-separated species, including electron/hole (e- /h+ ) polarons and spin-coupled triplet states, as uniquely confirmed by the light-induced electron paramagnetic resonance spectroscopy. The relative population of the e- /h+ pairs and triplet species depends on the solvent polarity, featuring a substantial stabilization of the triplet state in a non-polar environment (benzene). The electronic properties of the photoexcited Asp-CDs emerge from their spatial organization being interpreted as multi-layer assemblies containing a hydrophobic carbonaceous core and a hydrophilic oxygen and nitrogen functionalized surface. The system properties are dissected theoretically by density functional theory in combination with molecular dynamics simulations on quasi-spherical assemblies of size-variant flakelike model systems, revealing the importance of size dependence and interlayer effects. The formation of the spin-separated states in Asp-CDs enables the photoproduction of hydrogen peroxide (H2 O2 ) from water and water/2-propanol mixture via a water oxidation reaction.

Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure-activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

Augmenting the Performance of Hydrogenase for Aerobic Photocatalytic Hydrogen Evolution via Solvent Tuning

This work showcases the performance of [NiFeSe] hydrogenase from Desulfomicrobium baculatum for solar-driven hydrogen generation in a variety of organic-based deep eutectic solvents. Despite its well-known sensitivity towards air and organic solvents, the hydrogenase shows remarkable performance under an aerobic atmosphere in these solvents when paired with a TiO2 photocatalyst. Tuning the water content further increases hydrogen evolution activity to a TOF of 60±3 s-1 and quantum yield to 2.3±0.4 % under aerobic conditions, compared to a TOF of 4 s-1 in a purely aqueous solvent. Contrary to common belief, this work therefore demonstrates that placing natural hydrogenases into non-natural environments can enhance their intrinsic activity beyond their natural performance, paving the way for full water splitting using hydrogenases.

Supramolecular Chemistry of Carbon-Based Dots Offers Widespread Opportunities

Carbon dots are an emerging class of nanomaterials that has recently attracted considerable attention for applications that span from biomedicine to energy. These photoluminescent carbon nanoparticles are defined by characteristic sizes of <10 nm, a carbon-based core and various functional groups at their surface. Although the surface groups are widely used to establish non-covalent bonds (through electrostatic interactions, coordinative bonds, and hydrogen bonds) with various other (bio)molecules and polymers, the carbonaceous core could also establish non-covalent bonds (ππ stacking or hydrophobic interactions) with π-extended or apolar compounds. The surface functional groups, in addition, can be modified by various post-synthetic chemical procedures to fine-tune the supramolecular interactions. Our contribution categorizes and analyzes the interactions that are commonly used to engineer carbon dots-based materials and discusses how they have allowed preparation of functional assemblies and architectures used for sensing, (bio)imaging, therapeutic applications, catalysis, and devices. Using non-covalent interactions as a bottom-up approach to prepare carbon dots-based assemblies and composites can exploit the unique features of supramolecular chemistry, which include adaptability, tunability, and stimuli-responsiveness due to the dynamic nature of the non-covalent interactions. It is expected that focusing on the various supramolecular possibilities will influence the future development of this class of nanomaterials.

Lignocellulosic biomass valorization via bio-photo/electro hybrid catalytic systems

Lignocellulosic biomass valorization is regarded as a promising approach to alleviate energy crisis and achieve carbon neutrality. Bioactive enzymes have attracted great attention and been commonly applied for biomass valorization owing to their high selectivity and catalytic efficiency under environmentally benign reaction conditions. Same as biocatalysis, photo-/electro-catalysis also happens at mild conditions (i.e., near ambient temperature and pressure). Therefore, the combination of these different catalytic approaches to benefit from their resulting synergy is appealing. In such hybrid systems, harness of renewable energy from the photo-/electro-catalytic compartment can be combined with the unique selectivity of biocatalysts, therefore providing a more sustainable and greener approach to obtain fuels and value-added chemicals from biomass. In this review, we firstly introduce the pros/cons, classifications, and the applications of photo-/electro-enzyme coupled systems. Then we focus on the fundamentals and comprehensive applications of the most representative biomass-active enzymes including lytic polysaccharide monooxygenases (LPMOs), glucose oxidase (GOD)/dehydrogenase (GDH) and lignin peroxidase (LiP), together with other biomass-active enzymes in the photo-/electro- enzyme coupled systems. Finally, we propose current deficiencies and future perspectives of biomass-active enzymes to be applied in the hybrid catalytic systems for global biomass valorization.

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.

Repurposing Photosensitizer Proteins Through Genetic Code Expansion to Facilitate Photo-Biocatalysis

Artificial photoenzymes with noncanonical photo-redox cofactors have paved the way for enzyme rational design and the creation of new-to-nature biocatalysts. Genetically encoded photo-redox cofactors endow photoenzymes with enhanced or novel activities that catalyze numerous transformations with high efficiency. Herein, we describe a protocol of repurposing photosensitizer proteins (PSP) through genetic code expansion to facilitate multiple photocatalytic conversions including photo-activated dehalogenation of aryl halides, CO₂ to CO and CO₂ to formic acid reduction. The methods for expression, purification, and characterization of the PSP are detailed. The installation of the catalytic modules and the utilization of PSP-based artificial photoenzymes for photoenzymatic CO₂ reduction and dehalogenation are also described.

Natural xylose-derived carbon dots towards efficient semi-artificial photosynthesis

Photosynthesis by plants stores sunlight into chemicals and drives CO2 fixation into sugars with low biomass conversion efficiency due to the unoptimized solar spectrum utilization and various chemical conversion possibilities that follow H2O oxidation. Expanding the solar spectrum utilization and optimizing the charge transfer pathway of photosynthesis is critical to improving the conversion efficiency. Here, a group of carbon dots (CDs) with distinct content of sp2 CC domain are prepared by one-step carbonization of natural xylose, which penetrated natural chloroplasts and integrated with the grana thylakoid to promote in vitro photosynthesis. Structural characterization and electrochemical results reveal the positive impact of graphitization degree on the electron transport capacity of CDs. Classic Hill reaction and ATP production demonstrate the enhanced photosynthetic activity resulting from the CDs-mediated electron transfer of photosystem II. In-depth studies of the structure-function relationship prove the synergistic effect of intensified biotic-abiotic interaction between CDs and chloroplast, lower charge transfer resistance, and extended light absorption. This work posts a promising method to optimize electron transport and improve natural photosynthesis using artificial interventions.

Ultrafast insights into full-colour light-emitting C-Dots

Designing carbon dots (C-Dots) in a controlled way requires a profound understanding of their photophysical properties, such as the origin of their fluorescence and excitation wavelength-dependent emission properties, which has been a perennial problem in the last few decades. Herein, we synthesized three different C-Dots (blue, green, and red-emitting C-Dots) from the same starting materials via a hydrothermal method and separated them by silica column chromatography. All the purified C-Dots exhibited three different emission maxima after a certain range of different excitations, showing a high optical uniformity in their emission properties. It was also observed that the average distributions of the particle size in all the C-Dots were the same with a typical size of 4 nm and the same interplanar d spacing of ∼0.21 nm. Here, we tried to establish a well-defined conclusive answer to the puzzling optical properties of C-Dots via successfully investigating the carrier dynamics of their core and surface state with a myriad use of steady-state, time-resolved photoluminescence, and ultrafast transient absorbance spectroscopy techniques. The ultrafast charge-carrier dynamics of the core and surface state clearly indicated that the graphitic nitrogen in the core state and the oxygen-containing functional group in the surface state predominately contribute to controlling their wide range of emission properties. We believe that these findings will give the C-Dots their own designation in the fluorophore world and create a new avenue for designing and developing C-Dot-based new architectures.

Semiconductor quantum dots for photodynamic therapy: Recent advances

Photodynamic therapy is a promising cancer treatment that induces apoptosis as a result of the interactions between light and a photosensitizing drug. Lately, the emergence of biocompatible nanoparticles has revolutionized the prospects of photodynamic therapy (PDT) in clinical trials. Consequently, a lot of research is now being focused on developing non-toxic, biocompatible nanoparticle-based photosensitizers for effective cancer treatments using PDT. In this regard, semiconducting quantum dots have shown encouraging results. Quantum dots are artificial semiconducting nanocrystals with distinct chemical and physical properties. Their optical properties can be fine-tuned by varying their size, which usually ranges from 1 to 10 nm. They present many advantages over conventional photosensitizers, mainly their emission properties can be manipulated within the near IR region as opposed to the visible region by the former. Consequently, low intensity light can be used to penetrate deeper tissues owing to low scattering in the near IR region. Recently, successful reports on imaging and PDT of cancer using carbon (carbon, graphene based) and metallic (Cd based) based quantum dots are promising. This review aims to summarize the development and the status quo of quantum dots for cancer treatment.

Artificial photosynthesis systems for solar energy conversion and storage: platforms and their realities

In natural photosynthesis, photosynthetic organisms such as green plants realize efficient solar energy conversion and storage by integrating photosynthetic components on the thylakoid membrane of chloroplasts. Inspired by natural photosynthesis, researchers have developed many artificial photosynthesis systems (APS's) that integrate various photocatalysts and biocatalysts to convert and store solar energy in the fields of resource, environment, food, and energy. To improve the system efficiency and reduce the operation cost, reaction platforms are introduced in APS's since they allow for great stability and continuous processing. A systematic understanding of how a reaction platform affects the performance of artificial photosynthesis is conducive for designing an APS with superb solar energy utilization. In this review, we discuss the recent APS's researches, especially those confined on/in platforms. The importance of different platforms and their influences on APS's performance are emphasized. Generally, confined platforms can enhance the stability and repeatability of both photocatalysts and biocatalysts in APS's as well as improve the photosynthetic performance due to the proximity effect. For functional platforms that can participate in the artificial photosynthesis reactions as active parts, a high integration of APS's components on/in these platforms can lead to efficient electron transfer, enhanced light-harvesting, or synergistic catalysis, resulting in superior photosynthesis performance. Therefore, the integration of APS's components is beneficial for the transfer of substrates and photoexcited electrons in artificial photosynthesis. We finally summarize the current challenges of APS's development and further efforts on the improvement of APS's.

Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO₂ reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots (a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (-NHMe₂⁺) and negative (-COO^-) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO₂ reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO₂ reduction via direct electron transfer with >90% Faradaic efficiency and -250 μA cm^-2 at -0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (-NHMe₂⁺-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO₂ reduction (turnover frequency: 3.5 × 10³ h^-1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO₂ reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.

Polymer Dots as Photoactive Membrane Vesicles for [FeFe]-Hydrogenase Self-Assembly and Solar-Driven Hydrogen Evolution

A semiartificial photosynthesis approach that utilizes enzymes for solar fuel production relies on efficient photosensitizers that should match the enzyme activity and enable long-term stability. Polymer dots (Pdots) are biocompatible photosensitizers that are stable at pH 7 and have a readily modifiable surface morphology. Therefore, Pdots can be considered potential photosensitizers to drive such enzyme-based systems for solar fuel formation. This work introduces and unveils in detail the interaction within the biohybrid assembly composed of binary Pdots and the HydA1 [FeFe]-hydrogenase from Chlamydomonas reinhardtii. The direct attachment of hydrogenase on the surface of toroid-shaped Pdots was confirmed by agarose gel electrophoresis, cryogenic transmission electron microscopy (Cryo-TEM), and cryogenic electron tomography (Cryo-ET). Ultrafast transient spectroscopic techniques were used to characterize photoinduced excitation and dissociation into charges within Pdots. The study reveals that implementation of a donor-acceptor architecture for heterojunction Pdots leads to efficient subpicosecond charge separation and thus enhances hydrogen evolution (88 460 μmol_H2·g_H2ase^-1·h^-1). Adsorption of [FeFe]-hydrogenase onto Pdots resulted in a stable biohybrid assembly, where hydrogen production persisted for days, reaching a TON of 37 500 ± 1290 in the presence of a redox mediator. This work represents an example of a homogeneous biohybrid system combining polymer nanoparticles and an enzyme. Detailed spectroscopic studies provide a mechanistic understanding of light harvesting, charge separation, and transport studied, which is essential for building semiartificial photosynthetic systems with efficiencies beyond natural and artificial systems.

Carbon Quantum Dots from Pomelo Peel as Fluorescence Probes for “Turn-Off–On” High-Sensitivity Detection of Fe3+ and L-Cysteine

This study designed a “turn-off–on” fluorescence analysis method based on carbon quantum dots (CQDs) to detect metal ions and amino acids in real sample systems. CQDs were derived from green pomelo peel via a one-step hydrothermal process. The co-doped CQDs with N and S atoms imparted excellent optical properties (quantum yield = 17.31%). The prepared CQDs could be used as fluorescent “turn-off” probes to detect Fe³⁺ with a limit of detection of 0.086 µM, a linear detection range of 0.1–160 µM, and recovery of 83.47–106.53% in water samples. The quenched CQD fluorescence could be turned on after adding L-cysteine (L-Cys), which allowed detection of L-Cys with a detection limit of 0.34 µM and linear range of 0.4–85 µM. Recovery of L-Cys in amino acid beverage was 87.08–122.74%. Visual paper-based testing strips and cellulose/CQDs composite hydrogels could be also used to detect Fe³⁺ and L-Cys.

Folic Acid-Modified Cerium-Doped Carbon Dots as Photoluminescence Sensors for Cancer Cells Identification and Fe(III) Detection

Carbon dots (CDs) are a new class of carbon-based luminescence materials with fascinating properties. They have been given great expectations on superseding traditional semiconductor quantum dots due to their good dispersity and stability, relatively low toxicity, superior resistance to photobleaching, and excellent biocompatibility. The diversified luminescence properties of CDs are largely due to the synthetic strategies and precursors. In view of those described above, this study has explored the possibility to establish a facile one-step hydrothermal method for the one-pot synthesis of folic acid-modified cerium-doped CDs (Ce-CDs-FA), which could be further utilized as a sensitive fluorescent nanoprobe for biosensing. This investigation demonstrates that the Ce-CDs-FA nanocomposites have nice biocompatibility and bright fluorescent properties, which can be readily utilized to detect cancer cells through recognizing overexpressing folate receptors by virtue of folic acid. Meanwhile, it is noted that the Fe3+ ion can actualize a specific and hypersensitive quenching effect for these Ce-CDs-FA nanocomposites, which can be further explored for special ion recognition, including iron ions. It raises the possibility that the as-prepared Ce-CDs-FA nanocomposites could be extended as a dual fluorescence sensor for targeted cell imaging and Fe3+ ion detection.

Visual Monitoring of Nucleic Acid Dynamic Structures during Cellular Ferroptosis Using Rationally Designed Carbon Dots with Robust Anti-Interference Ability to Reactive Oxygen Species

Ferroptosis triggered by an iron-dependent accumulation of lipid reactive oxygen species (ROS) has drawn widespread attention. Directly visualizing the dynamic structures of nucleic acids during the ferroptosis of cells is of great importance considering their vital roles in numerous biological functions. However, direct imaging remains challenging, largely due to the extremely high concentrations of ROS generated during ferroptosis, which can affect the imaging of nucleic acid targeted fluorescent probes. To overcome this challenge, nucleic acid-responsive carbon dots (CDs) providing favorable optical properties together with high chemical stability were synthesized. Furthermore, the CDs penetrated the cell membrane quickly and accumulated in the nuclei of cells. The robust anti-interference ability to ROS allows the CDs to visualize the dynamic structures of nucleic acids during ferroptosis. Moreover, the CDs were successfully employed in the imaging of nucleic acids during cell division. The nuclei-targeting CDs show great potential as a powerful tool for imaging nuclei in ferroptosis-related biological and clinical research.

Ultrafast Visible-Light-Induced ATRP in Aqueous Media with Carbon Quantum Dots as the Catalyst and Its Application for 3D Printing

Photoinduced atom transfer radical polymerization (ATRP) has been proved to be a versatile technique for polymer network formation. However, the slow polymerization rates of typical ATRP limited its application in the field of additive manufacturing (3D printing). In this work, we introduced carbon quantum dots (CQDs) for the first time to the ATRP in aqueous media and developed an ultrafast visible-light-induced polymerization system. After optimization, the polymerization could achieve a high monomer conversion (>90%) within 1 min, and the polydispersity index (PDI) of the polymer was lower than 1.25. This system was then applied as the first example of ATRP for the 3D printing of hydrogel through digital light processing (DLP), and the printed object exhibited good dimensional accuracy. Additionally, the excellent and stable optical properties of CQDs also provided interesting photoluminescence capabilities to the printed objects. We deduce this ATRP mediated 3D printing process would provide a new platform for the preparation of functional and stimuli-responsive hydrogel materials.

Emerging Strategies in Enhancing Singlet Oxygen Generation of Nano-Photosensitizers Toward Advanced Phototherapy

The great promise of photodynamic therapy (PDT) has thrusted the rapid progress of developing highly effective photosensitizers (PS) in killing cancerous cells and bacteria. To mitigate the intrinsic limitations of the classical molecular photosensitizers, researchers have been looking into designing new generation of nanomaterial-based photosensitizers (nano-photosensitizers) with better photostability and higher singlet oxygen generation (SOG) efficiency, and ways of enhancing the performance of existing photosensitizers. In this paper, we review the recent development of nano-photosensitizers and nanoplasmonic strategies to enhance the SOG efficiency for better PDT performance. Firstly, we explain the mechanism of reactive oxygen species generation by classical photosensitizers, followed by a brief discussion on the commercially available photosensitizers and their limitations in PDT. We then introduce three types of new generation nano-photosensitizers that can effectively produce singlet oxygen molecules under visible light illumination, i.e., aggregation-induced emission nanodots, metal nanoclusters (< 2 nm), and carbon dots. Different design approaches to synthesize these nano-photosensitizers were also discussed. To further enhance the SOG rate of nano-photosensitizers, plasmonic strategies on using different types of metal nanoparticles in both colloidal and planar metal-PS systems are reviewed. The key parameters that determine the metal-enhanced SOG (ME-SOG) efficiency and their underlined enhancement mechanism are discussed. Lastly, we highlight the future prospects of these nanoengineering strategies, and discuss how the future development in nanobiotechnology and theoretical simulation could accelerate the design of new photosensitizers and ME-SOG systems for highly effective image-guided photodynamic therapy.

The Role of N and S Doping on Photoluminescent Characteristics of Carbon Dots from Palm Bunches for Fluorimetric Sensing of Fe3+ Ion

This work aims to enhance the value of palm empty fruit bunches (EFBs), an abundant residue from the palm oil industry, as a precursor for the synthesis of luminescent carbon dots (CDs). The mechanism of fIuorimetric sensing using carbon dots for either enhancing or quenching photoluminescence properties when binding with analytes is useful for the detection of ultra-low amounts of analytes. This study revealed that EFB-derived CDs via hydrothermal synthesis exceptionally exhibited luminescence properties. In addition, surface modification for specific binding to a target molecule substantially augmented their PL characteristics. Among the different nitrogen and sulfur (N and S) doping agents used, including urea (U), sulfate (S), p-phenylenediamine (P), and sodium thiosulfate (TS), the results showed that PTS-CDs from the co-doping of p-phenylenediamine and sodium thiosulfate exhibited the highest PL properties. From this study on the fluorimetric sensing of several metal ions, PTS-CDs could effectively detect Fe3+ with the highest selectivity by fluorescence quenching to 79.1% at a limit of detection (LOD) of 0.1 µmol L−1. The PL quenching of PTS-CDs was linearly correlated with the wide range of Fe3+ concentration, ranging from 5 to 400 µmol L−1 (R2 = 0.9933).

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Light-Induced Hypoxia in Carbon Quantum Dots and Ultrahigh Photocatalytic Efficiency

Carbon quantum dots (CQDs) represent a class of carbon materials exhibiting photoresponse and many potential applications. Here, we present a unique property that dissolved CQDs capture large amounts of molecular oxygen from the air, the quantity of which can be controlled by light irradiation. The O₂ content can be varied between a remarkable 1 wt % of the CQDs in the dark to nearly half of it under illumination, in a reversible manner. Moreover, O₂ depletion enhances away from the air-solution interface as the nearby CQDs quickly regain them from the air, creating a pronounced concentration gradient in the solution. We elucidate the role of the CQD functional groups and show that excitons generated under light are responsible for their tunable adsorbed-oxygen content. Because of O₂ enrichment, the photocatalytic efficiency of the CQDs toward oxidation of benzylamines in the air is the same as under oxygen flow and far higher than the existing photocatalysts. The findings should encourage the development of a new class of oxygen-enricher materials and air as a sustainable oxidant in chemical transformations.