DNA-Functionalized Nanoceria for Probing Oxidation of Phosphorus Compounds

Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

Hydrogen spillover and substrate-support hydrogen bonding mediate hydrogenation of phenol catalyzed by palladium on reducible metal oxides

Substrate-support interactions play an important role in the catalytic hydrogenation of phenolic compounds by ceria-supported palladium (Pd/CeO2). Here, we combine surface contrast solution NMR methods and reaction kinetic assays to investigate the role of substrate-support interactions in phenol (PhOH) hydrogenation catalyzed by titania-supported palladium (Pd/TiO2). We show that PhOH adsorbs on the catalyst via a weak hydrogen-bonding interaction between the -OH group of the substrate and one oxygen atom on the support. Interestingly, we observe that the addition of 20 mM inorganic phosphate results in a ∼2-fold destabilization of the PhOH-support interaction and a corresponding ∼2-fold inhibition of the catalytic reaction, suggesting an active role of the PhOH-TiO2 hydrogen bond in catalysis. A comparison of the data measured here with the results previously reported for a Pd/CeO2 catalyst indicates that the efficiency of the Pd-supported catalysts is correlated to the amount of PhOH hydrogen bonded to the metal oxide support. Since CeO2 and TiO2 have similar ability to uptake activated hydrogen from a noble metal site, these data suggest that hydrogen spillover is the main mechanism by which Pd-activated hydrogens are shuttled to the PhOH adsorbed on the surface of the support. Consistent with this hypothesis, Pd supported on a non-reducible metal oxide (silica) displays negligible hydrogenation activity. Therefore, we conclude that basic and reducible metal oxides are active supports for the efficient hydrogenation of phenolic compounds due to their ability to hydrogen bond to the substrate and mediate the addition of the activated hydrogens to the adsorbed aromatic ring.

Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

Characterization of dynamic processes occurring at the nanoparticle (NP) surface is crucial for developing new and more efficient NP catalysts and materials. Thus, a vast amount of research has been dedicated to developing techniques to characterize sorption equilibria. Over recent years, solution NMR spectroscopy has emerged as a preferred tool for investigating ligand-NP interactions. Indeed, due to its ability to probe exchange dynamics over a wide range of timescales with atomic resolution, solution NMR can provide structural, kinetic, and thermodynamic information on sorption equilibria involving multiple adsorbed species and intermediate states. In this contribution, we review solution NMR methods for characterizing ligand-NP interactions, and provide examples of practical applications using these methods as standalone techniques. In addition, we illustrate how the integrated analysis of several NMR datasets was employed to elucidate the role played by support-substrate interactions in mediating the phenol hydrogenation reaction catalyzed by ceria-supported Pd nanoparticles.

Nanoceria, the versatile nanoparticles: Promising biomedical applications

Nanotechnology has been a boon for the biomedical field due to the freedom it provides for tailoring of pharmacokinetic properties of different drug molecules. Nanomedicine is the medical application of nanotechnology for the diagnosis, treatment and/or management of the diseases. Cerium oxide nanoparticles (CNPs) are metal oxide-based nanoparticles (NPs) which possess outstanding reactive oxygen species (ROS) scavenging activities primarily due to the availability of "oxidation switch" on their surface. These NP have been found to protect from a number of disorders with a background of oxidative stress such as cancer, diabetes etc. In fact, the CNPs have been found to possess the environment-dependent ROS modulating properties. In addition, the inherent catalase, SOD, oxidase, peroxidase and phosphatase mimetic properties of CNPs provide them superiority over a number of NPs. Further, chemical reactivity of CNPs seems to be a function of their surface chemistry which can be precisely tuned by defect engineering. However, the contradictory reports make it necessary to critically evaluate the potential of CNPs, in the light of available literature. The review is aimed at probing the feasibility of CNPs to push towards the clinical studies. Further, we have also covered and censoriously discussed the suspected negative impacts of CNPs before making our way to a consensus. This review aims to be a comprehensive, authoritative, critical, and accessible review of general interest to the scientific community.

Nanostructured Ceria: Biomolecular Templates and (Bio)applications

Ceria (CeO2) nanostructures are well-known in catalysis for energy and environmental preservation and remediation. Recently, they have also been gaining momentum for biological applications in virtue of their unique redox properties that make them antioxidant or pro-oxidant, depending on the experimental conditions and ceria nanomorphology. In particular, interest has grown in the use of biotemplates to exert control over ceria morphology and reactivity. However, only a handful of reports exist on the use of specific biomolecules to template ceria nucleation and growth into defined nanostructures. This review focusses on the latest advancements in the area of biomolecular templates for ceria nanostructures and existing opportunities for their (bio)applications.

Nanomaterial and Aptamer-Based Sensing: Target Binding versus Target Adsorption Illustrated by the Detection of Adenosine and ATP on Metal Oxides and Graphene Oxide

Target molecule-induced desorption of aptamer probes from nanomaterials has been a very popular sensing method, taking advantage of the fluorescence quenching or catalytic activity of nanomaterials for signal generation. While it is generally conceived that aptamers desorb due to binding to target molecules, in this work, we examined the effect of competitive target adsorption. From five metal oxide nanoparticles including CeO₂, ZnO, NiO, Fe₃O₄, and TiO₂, only ATP was able to induce desorption of its aptamer. Adenosine could not, even though it had an even higher affinity than ATP to the aptamer. The same conclusion was also observed with a random DNA that cannot bind ATP, indicating that the desorption of DNA was due to competitive adsorption of ATP instead of aptamer binding. On graphene oxide, however, adenosine produced slightly more aptamer desorption than ATP under most of the conditions, and this can be partially attributed to the weaker interaction of negatively charged ATP with negatively charged graphene oxide. For such surface-based biosensors, it is recommended that a nonaptamer control DNA be tested side-by-side to ensure the sensing mechanism to be related to aptamer binding instead of target adsorption.

Effect of proteins on the oxidase-like activity of CeO2 nanozymes for immunoassays

Protein adsorption inhibits the oxidase-like activity of CeO₂ nanoparticles. Coating a partial shell of silica on CeO₂ and subsequent conjugation of antibodies allow highly sensitive and selective detection of fenitrothion.

A Multichannel Pattern-Recognition-Based Protein Sensor with a Fluorophore-Conjugated Single-Stranded DNA Set

Recently, pattern-recognition-based protein sensing has received considerable attention because it offers unique opportunities that complement more conventional antibody-based detection methods. Here, we report a multichannel pattern-recognition-based sensor using a set of fluorophore-conjugated single-stranded DNAs (ssDNAs), which can detect various proteins. Three different fluorophore-conjugated ssDNAs were placed into a single microplate well together with a target protein, and the generated optical response pattern that corresponds to each environment-sensitive fluorophore was read via multiple detection channels. Multivariate analysis of the resulting optical response patterns allowed an accurate detection of eight different proteases, indicating that fluorescence signal acquisition from a single compartment containing a mixture of ssDNAs is an effective strategy for the characterization of the target proteins. Additionally, the sensor could identify proteins, which are potential targets for disease diagnosis, in a protease and inhibitor mixture of different composition ratios. As our sensor benefits from simple construction and measurement procedures, and uses accessible materials, it offers a rapid and simple platform for the detection of proteins.

Promotion and Inhibition of the Oxidase‐Mimicking Activity of Nanoceria by Phosphate, Polyphosphate, and DNA

Nanoceria (CeO2 nanoparticles) is an extensively studied nanozyme with interesting oxidase‐mimicking activity. As they can work in the absence of toxic and unstable H2O2, CeO2 nanoparticles have been widely used in biosensing. CeO2 nanoparticles often encounter phosphate‐containing molecules that can affect their catalytic activity, and various reports exist in the literature showing both promoted and inhibited activity. In this work, we systematically studied five types of phosphate: orthophosphate, pyrophosphate, triphosphate, trimetaphosphate, and a polyphosphate with 25 phosphate units (Pi25). In addition, DNA oligonucleotides of various length and sequence. DNA was included as they contain a phosphate backbone that can strongly adsorb on nanoceria. We observed that a high concentration of DNA in acetate buffer inhibited activity, whereas a low concentration of DNA in phosphate buffer increased activity. The change of activity was also related to the type of substrate and related to the aggregation of CeO2. These discoveries provide an important understanding for the further use of CeO2 nanoparticles in biosensor development, materials science, and nanotechnology.

Electronic structure engineering and biomedical applications of low energy-excited persistent luminescence nanoparticles

Persistent luminescence nanoparticles (PLNPs) are new luminescent materials that can store the excitation energy quickly and persistently emit it after ceasing excitation sources. Due to the advantages of long-lasting luminescence without constant excitation, PLNPs have been widely used in biomedical applications. Visible light excitable PLNPs (VPLNPs) and near-infrared excitable PLNPs (NPLNPs) are two kinds of novel and promising PLNPs. Compared to conventional PLNPs, VPLNPs and NPLNPs have the characteristics of low tissue damage, deep tissue penetration, and high signal-to-noise ratio. With these special features, they have great potential in applications such as long-term tracing, deep-tissue bioimaging, and precise treatment. In this review, we introduce the common strategy of constructing VPLNPs and NPLNPs based on electronic structure engineering and the applications of VPLNPs and NPLNPs in biomedicine. This review article aims to offer valuable information about the progress and development direction of VPLNPs and NPLNPs, promoting more applications in biomedicine, materials science, energy engineering, and environmental technologies in the future.

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.

Ceria decorated porous diatom-xerogel as an effective adsorbent for the efficient removal of Eriochrome Black T

Diatomaceous earth or diatom (DE) are naturally available and low cost micro particles with distinct porous structure were used as an adsorbent for the removal of a hazardous dye, Eriochrome Black T (EBT). The surface modification of these DE were performed by sol-gel and hydrothermal methods to obtain a series of adsorbents such as diatom-ceria (DC), diatom-silica xerogel (DX), and diatom-silica xerogel-ceria (DXC). A cauliflower like morphology structure of ceria was observed on DE and DX. The adsorption performance of EBT was conducted by varying various parameters such as pH, adsorbent dosage, initial concentration, contact time and ionic strength. The materials DE, DC, DX and DXC showed the EBT removal efficiencies of 52, 77, 20, and 93%, respectively. The maximum adsorption capacity (qm) of DE, DC, DX and DXC was found to be 13.83, 23.64, 0.2 and 47.02 mgg^-1 for the adsorption of EBT, respectively. The selectivity of EBT towards DXC was evaluated by treating a mixture of anionic dyes. The dye removal experiments was performed in presence of inorganic salts, however the presence of these salts did not affect the removal efficiency of DXC. Furthermore, the reusability of DXC was studied by recycling it up to 5 times and even at 5th cycle a removal efficiency of ∼66.8% was found. Thus, these studies demonstrate that the DXC material could be a promising candidate for the removal of EBT via adsorption for real time application in water treatment.

Adsorption Promoted Aggregation-Induced Emission Showing Strong Dye Lateral Interactions

Aggregation-induced emission (AIE) is a powerful method to produce fluorescence for a diverse range of applications. While most previous work induced aggregation by change of solvent, ionic strength, pH, or self-assembly, we herein explored adsorption-induced aggregation using 4,4'-(hydrazine-1,2-diylidene bis(methanylylidene)) bis(3-hydroxybenzoic acid) (HDBB) as an AIE luminogen. HDBB is known to aggregate with AIE at low pH but not at neutral pH, and its aggregation facilitates excited state intramolecular proton transfer for enhanced emission. Using a nonquenching nanomaterial, Y₂O₃ nanoparticles, HDBB showed sevenfold fluorescence increase at pH 7.0. Fluorescence lifetime showed that HDBB was in the aggregated state in the presence of Y₂O₃. For comparison, a fluorescent porphyrin compound showed that adsorption caused quenching after mixing with Y₂O₃, whereas other dyes such as fluorescein, calcein, and rhodamine B failed to be adsorbed by Y₂O₃. Adsorption did not follow a Langmuir isotherm, but it showed strong lateral HDBB interactions because adsorption was only achieved with a high concentration of HDBB. Adsorption was inhibited by salt and by phosphate, indicating the importance of electrostatic and metal-binding interactions. Comparisons were made with other nanomaterials, where graphene oxide and CeO₂ quenched HDBB and a cationic liposome also enhanced its emission, although with a less red-shifted peak wavelength. This study provides a simple method to induce aggregation of an AIE dye and its aggregation in turn-enhanced adsorption.

Nanoceria as a DNase I mimicking nanozyme

We herein communicate the DNase I like activity of nanoceria (CeO2 nanoparticles). Both CeO2 and DNase I cleave polyadenine (poly-A) DNA down to ∼5-mer fragments as the major products, although further cleavage to even shorter fragments was observed with CeO2. Mass spectrometry indicates a hydrolytic cleavage mechanism instead of oxidative cleavage.

Effects of Small Molecules on DNA Adsorption by Gold Nanoparticles and a Case Study of Tris(2-carboxyethyl)phosphine (TCEP)

DNA-functionalized gold nanoparticles (AuNPs) often encounter various small molecules and ions such as backfilling agents, bifunctional cross-linkers, stabilizers, and molecules from biological fluids both during and after the DNA conjugation process. Small molecules and ions can influence the stability and property of the conjugate, but such interactions are yet to be fully explored. In this work, eight important molecules were studied and compared, including tris(2-carboxyethyl)phosphine hydrochloride (TCEP), 3-(2-pyridyldithio)propionic acid N-hydroxysuccinimide ester (SPDP), 4-maleimidobutyric acid N-hydroxysuccinimide ester (GMBS), 6-hydroxy-1-hexanethiol (MCH), l-glutathione (GSH), bromide (Br^-), bis(p-sulfonatophenyl)phenylphosphine (BSPP), and thiocyanate (SCN^-). Depending on the size, charge, and adsorption affinity on the AuNPs, they can either stabilize or destabilize the AuNPs. Their ability to displace thiolated DNA from AuNPs follows the order of MCH > SPDP > GSH > SCN^- > TCEP > Br^- > BSPP > GMBS. BSPP has the best stabilization effect for the colloidal stability of AuNPs, while it does not displace the adsorbed DNA. TCEP can be adsorbed on AuNPs and enhance the adsorption of A/C rich DNA in low-salt conditions. This work indicates that the effects of small molecules and ions cannot be ignored when studying the DNA-functionalized AuNPs, which ensures optimal applications and correct interpretation of the data.

The Arsenic-Binding Aptamer Cannot Bind Arsenic: Critical Evaluation of Aptamer Selection and Binding

An arsenic-binding aptamer named Ars-3 was reported in 2009, and it has been used for detection of As(III) in more than two dozen papers. In this work, we performed extensive binding assays using isothermal titration calorimetry, various DNA-staining dyes, and gold nanoparticles. By carefully comparing Ars-3 and a few random control DNA sequences, no specific binding of As(III) was observed in each case. Therefore, we conclude that Ars-3 cannot bind As(III). Possible reasons for some of the previously reported binding and detection were speculated to be related to the adsorption of As(III) onto gold surfaces, which were used in many related sensor designs, and As(III)/Au interactions were not considered before. The selection data in the original paper were then analyzed in terms of sequence alignment, secondary structure prediction, and dissociation constant measurement. These steps need rigorous testing before confirming specific binding of newly selected aptamers. This study calls for attention to the gap between aptamer selection and biosensor design, and the gap needs to be filled by careful binding assays to further the growth of the aptamer field.