Nanomaterial-based enzyme mimics (nanozymes) are currently a new forefront of chemical research. However, the application of nanozymes is limited by its low catalytic activity and low turnover numbers. Cerium dioxide nanoparticles (nanoceria) is among the few with oxidase activity. Herein, we report an interesting finding addressing its limitations. The oxidase activity of nanoceria is improved by over 100-fold by fluoride capping, rendering it more close to real oxidases (e.g., cytochrome P450). The turnover number reached 700 in 15 min, drastically improved from ~15 turnovers for the naked particles. The mechanism is attributed to surface charge modulation and facilitated electron transfer by Fcapping based on ζ-potential and free radical measurement. Ultrasensitive sensing of fluoride was achieved with a detection limit of 0.64 µM F -in water and in toothpastes, while no other tested anions can achieve the activity enhancement.Developing nanomaterial-based enzyme mimics, known as nanozymes, is a new frontier in inorganic, materials and analytical chemistry. [1][2][3][4][5][6][7] Nanozymes are a subset of nanoparticle-based catalysts that catalyze enzyme-like reactions at ambient conditions. Nanozymes are attractive not only for their high stability and lowcost, but also for studying fundamental processes at nanoscale surfaces. 2 Many nanoparticles such as gold, 4,8,9 graphene oxide, 10,11 and various metal oxides 1,3,5,12-19 possess oxidase, peroxidase, and/or catalase like activities. It is interesting to note that while many peroxidase nanozymes were reported (i.e., using H2O2 as a cosubstrate), very few have oxidase activity. 9,[19][20][21][22][23] The best known examples are gold nanoparticles for glucose oxidation, 9 and CeO2 nanoparticles (nanoceria) that oxidize a diverse range of substrates. 20 Oxidase nanozymes are important since they do not require unstable H2O2 as a co-substrate.Nanoceria is a versatile nanozyme, displaying oxidase, 16,20 catalase, 24,25 superoxide dismutase, 26,27 and phosphatase 28 activities under different conditions. In 2009, Asati and co-workers first reported the oxidase-like activity of nanoceria. 20 Since then, nanoceria has become an attractive system receiving extensive studies. The catalytic activities of nanoceria were attributed to the mixed oxidation states of Ce 3+ and Ce 4+ , and related oxygen vacancies. 29,30 The role of redox states, 24 surface coating, 20 pH, 31 buffer composition, 32 and nucleotide triphosphate 16 were investigated. We hypothesize that surface chemistry plays a critical role in modulating substrate/product adsorption and electron transfer, thus influencing catalytic efficiency. A challenge in the nanozyme field is poor catalytic activity and low turnover, which is in sharp contrast to their highly efficient protein counterparts.

Attaching DNA to nanomaterials is the basis for DNA-directed assembly, sensing, and drug delivery using such hybrid materials. Poly-cytosine (poly-C) DNA is a high affinity ligand for four types of commonly used nanomaterials, including nanocarbons (graphene oxide and single-walled carbon nanotubes), transition metal dichalcogenides (MoS and WS ), metal oxides (Fe O and ZnO), and metal nanoparticles (Au and Ag). Compared to other homo-DNA sequences, poly-C DNA has the highest affinity for the first three types of materials. Using a diblock DNA containing a poly-C block to attach to surfaces, the target DNA was successfully hybridized to the other block on graphene oxide more efficiently than that containing a typical poly-A block, especially in the presence of non-specific background DNA, proteins, or surfactants. This work provides a simple solution for functionalizing nanomaterials with non-modified DNA and offers new insights into DNA biointerfaces.

Target-directed aptamer adsorption by gold nanoparticles (AuNPs) has been widely used to develop label-free colorimetric biosensors. However, the potential interactions between target molecules and AuNPs have not been considered, which may lead to misinterpretation of analytical results. In this work, the detection of dopamine, melamine, and K + was studied as model systems to address this problem. First, dopamine and two control molecules all induced the aggregation of citrate-capped AuNPs with apparent K d 's of 5.8 μM dopamine, 51.6 μM norepinephrine, and 142 μM tyramine. Isothermal titration calorimetry measured the aptamer K d to be 1.9 μM dopamine and 16.8 μM norepinephrine, whereas tyramine cannot bind. Surface enhanced Raman spectroscopy confirmed direct adsorption of dopamine, and the adsorbed dopamine inhibited the adsorption of DNA. Using a typical salt-induced colorimetric detection protocol, a similar color response was observed regardless of the sequence of DNA, indicating the observed color change reflected the adsorption of dopamine by the AuNPs instead of the binding of dopamine by the aptamer. For this label-free sensor to work, the interaction between the target molecule and AuNPs should be very weak, while dopamine represents an example of strong interactions. For the other two systems, the melamine detection did not reflect aptamer binding either but the K + detection did, suggesting melamine also strongly interacted with AuNPs, whereas K + had very weak interactions with AuNPs. Since each target molecule is different, such target/AuNP interactions need to be studied case-by-case to ensure the sensing mechanism.

Functionalization of a gold surface with DNA is often complicated by kinetic traps from unintended DNA base adsorption. Herein, we communicate that Br serves as a robust backfilling agent displacing selected DNA bases on gold. Traditional thiol backfillers are too strong, while even 300 mM Br is well tolerated. Conjugates prepared with Br hybridize 10-fold faster and resist DNA release with better colloidal stability yielding highly sensitive probes. From colorimetric and Raman assays, adsorption affinity ranks as F < T ≈ Cl < C < G ≈ Br < A < I, allowing Br to displace nonpoly-A sequences from gold. This well-controlled biointerface will impact biosensing, drug delivery, and directed assembly of nanomaterials.

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Sensing DNA in a complex sample matrix using the adsorption difference between DNA and proteins on nanomaterials.

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