Nanomolar hydrogen peroxide detection using horseradish peroxidase covalently linked to undoped nanocrystalline diamond surfaces

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen-vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.

Single-Nitrogen-Vacancy NMR of Amine-Functionalized Diamond Surfaces

Nuclear magnetic resonance (NMR) imaging with shallow nitrogen-vacancy (NV) centers in diamond offers an exciting route toward sensitive and localized chemical characterization at the nanoscale. Remarkable progress has been made to combat the degradation in coherence time and stability suffered by near-surface NV centers using suitable chemical surface termination. However, approaches that also enable robust control over adsorbed molecule density, orientation, and binding configuration are needed. We demonstrate a diamond surface preparation for mixed nitrogen- and oxygen-termination that simultaneously improves NV center coherence times for <10 nm-deep emitters and enables direct and recyclable chemical functionalization via amine-reactive cross-linking. Using this approach, we probe single NV centers embedded in nanopillar waveguides to perform ¹⁹F NMR sensing of covalently bound fluorinated molecules with detection on the order of 100 molecules. This work signifies an important step toward nuclear spin localization and structure interrogation at the single-molecule level.

Colorimetric assay based on horseradish peroxidase/reduced graphene oxide hybrid for sensitive detection of hydrogen peroxide in beverages

We reported a simple and sensitive colorimetric assay for detection of hydrogen peroxide (H₂O₂) based on the oxidation of 2,2׳-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) by UV-Vis spectroscopy method. The reduced graphene oxide (rGO) was prepared using green tea extract as bio-reducing and stabilizer agent and decorated by horseradish peroxidase (HRP). The surface of Au interface was modified with HRP-rGO hybrid. The formation of HRP-rGO hybrid was confirmed by cyclic voltammetry, scanning electron microscopy (SEM), energy-dispersive X-ray Spectroscopy (EDX) and Raman spectroscopy·H₂O₂ can be catalysed by HRP-rGO hybrid and converted into water and oxygen. The ABTS substrate takes up oxygen to form a green coloured product that has absorption peaks at 421, 655 nm and 737 nm. The colour development is linearly dependent on HRP in the range of 4-50 µg/L. The color of the green product solution is stable for 20 min. The absorption intensity is strongly related to the hydrogen peroxide concentration. The absorption intensity of the formed product scaled linearly with the hydrogen peroxide concentration in the ranges of 0.3-20 µM and 20-8000 µM with a detection limit of ≈15 nM could be achieved. The biosensor with excellent limit detection and wide linear ranges was adapted to monitor H₂O₂ in different beverages.

Expanded mesoporous silica-encapsulated ultrasmall Pt nanoclusters as artificial enzymes for tracking hydrogen peroxide secretion from live cells

Design of synthetic structures that possess the similar functions to natural enzymes held great promise in environmental detection and biomedical application. Herein, a new concept for the fabrication of solid-supported catalysts as peroxidase mimic have been proposed to realize high-catalytic activity and stability by utilizing expanded mesoporous silica (EMSN)-encapsulated Pt nanoclusters. Compared with PtNCs, the introduction of amino group modified EMSN would enrich H₂O₂ on the surface of PtNCs and increase the catalytic sites for H₂O₂ decomposition, which gave rise to the higher catalytic activity of EMSN-PtNCs over a broad pH range, especially in weakly acidic and neural solutions. This would facilitate their applications for real-time monitoring the secretion of H₂O₂ from living cancer cells stimulated by various anticancer drugs. Our findings not only pave the way to use porous matrix as the structural component for the design of the biomimetic catalysts, but also provide a simple and reliable platform to monitor H₂O₂ released from living cells in real time, which holds great potential for elucidating the biological roles of H₂O₂ and underlying molecular mechanisms of drug cytotoxicity as well as drug therapeutic effects.

Surface-immobilized and self-shaped DNA hydrogels and their application in biosensing

Surface-immobilized pure DNA hydrogels were formed using a surficial primer-induced strategy and adopted as scaffold materials for biosensing applications.

Hydrogels are of great interest in the field of biosensing for their good biocompatibility, plasticity, and capability of providing 3D scaffolds. Nevertheless, the application of hydrogels has not been linked with broad surface biosensing systems yet. To overcome the limitations, here for the first time, surface-immobilized pure DNA hydrogels were synthesized using a surficial primer-induced strategy and adopted for biosensing applications. The DNA hydrogel 3D scaffold is successfully constructed on a transparent ITO electrode, which facilitates both colourimetric and electrochemical measurements. Results show that the hydrogel is able to wrap enzymes solidly and exhibits favourable stability under different conditions. Owing to the free diffusion of the micromolecular targets throughout the hydrogel, while isolating the enzymes from the macromolecular interferences outside the hydrogel, the direct colourimetric and electrochemical detection of hydrogen peroxide and bilirubin in serum is achieved. The detection limit of hydrogen peroxide in serum is 22 nM by colourimetric analysis and 13 nM by electrochemical measurement. The detection limit of bilirubin is 32 nM, a favourable limit that could be used in jaundice diagnosis. In addition, the enzyme@hydrogel can be easily regenerated and the catalytic activity is retained for a few cycles, thus allowing the recycling of the hydrogel-based biosensing system. The successful integration of DNA hydrogels with surface biosensing systems will greatly expand the applications of hydrogels for diagnostic and environmental monitoring purposes.

Light-Triggered Release of Biomolecules from Diamond Nanowire Electrodes

The controlled release of biomolecules from a substrate surface is a challenging task. Photocleavable linkers appear as attractive candidates for light-triggered delivery. We show here the possibility of creating photoactivable diamond nanowire interfaces, from which molecules can be photochemically released upon irradiation at 365 nm for several minutes. The approach is based on the covalent modification of boron-doped diamond nanowires (BDD NWs) with o-nitrobenzyl containing ligands, to which different biomolecules can be attached via amide bond formation. The photodecomposition reaction and the subsequent release of small proteins such as lysozyme or enzymes such as horseradish peroxidase (HRP) are investigated using electrochemical impedance spectroscopy. Using a colorimetric assay, we demonstrate that, while complete cleavage of HRP was achieved upon irradiation for 10 min at 1 W cm(-2), this exposure time resulted in a partial loss of enzymatic activity.

Highly Chemiluminescent Graphene Oxide Hybrids Bifunctionalized by N-(Aminobutyl)-N-(Ethylisoluminol)/Horseradish Peroxidase and Sensitive Sensing of Hydrogen Peroxide

N-aminobutyl-N-ethylisoluminol and horseradish peroxidase bifunctionalized graphene oxide hybrids (ABEI-GO@HRP) were prepared through a facile and green strategy for the first time. The hybrids exhibited excellent chemiluminescence (CL) activity over a wide range of pH from 6.1 to 13.0 when reacted with H2O2, whereas ABEI functionalized GO had no CL emission at neutral pH and showed more than 2 orders of magnitude lower CL intensity than ABEI-GO@HRP at pH 13.0. Such strong CL emission from ABEI-GO@HRP was probably due to that HRP and GO facilitated the formation of O2(•-), - CO4(•2-), HO(•), and π-C═C(•) in the CL reaction, and GO as a reaction interface promoted the electron transfer of the radical-involved reaction. By virtue of ABEI-GO@HRP as a platform, an ultrasensitive, selective, and reagentless CL sensor was developed for H2O2 detection. The CL sensor exhibited a detection limit of 47 fM at physiological pH, which was more than 2 orders of magnitude lower than previously reported methods. This work reveals that bifunctionalization of GO by ABEI and HRP leads to excellent CL feature and enzyme selectivity, which can be used as an ideal platform for developing novel analytical methods.

Spheres-on-sphere silica microspheres as matrix for horseradish peroxidase immobilization and detection of hydrogen peroxide

Spheres-on-sphere (SOS) silica microspheres are employed as a matrix for horseradish peroxidase (HRP) immobilization. The SOS-COOH-HRP shows excellent catalytic performance and stability.

Succinimidyl ester surface chemistry: implications of the competition between aminolysis and hydrolysis on covalent protein immobilization

N-Hydroxysuccinimide (NHS) ester terminal groups are commonly used to covalently couple amine-containing biomolecules (e.g., proteins and peptides) to surfaces via amide linkages. This one-step aminolysis is often performed in buffered aqueous solutions near physiological pH (pH 6 to pH 9). Under these conditions, the hydrolysis of the ester group competes with the amidization process, potentially degrading the efficiency of the coupling chemistry. The work herein examines the efficiency of covalent protein immobilization in borate buffer (50 mM, pH 8.50) using the thiolate monolayer formed by the chemisorption of dithiobis (succinimidyl propionate) (DSP) on gold films. The structure and reactivity of these adlayers are assessed via infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), electrochemical reductive desorption, and contact angle measurements. The hydrolysis of the DSP-based monolayer is proposed to follow a reaction mechanism with an initial nucleation step, in contrast to a simple pseudo first-order reaction rate law for the entire reaction, indicating a strong dependence of the interfacial reaction on the packing and presence of defects in the adlayer. This interpretation is used in the subsequent analysis of IR-ERS kinetic plots which give a heterogeneous aminolysis rate constant, ka, that is over 3 orders of magnitude lower than that of the heterogeneous hydrolysis rate constant, kh. More importantly, a projection of these heterogeneous kinetic rates to protein immobilization suggests that under coupling conditions in which low protein concentrations and buffers of near physiological pH are used, proteins are more likely physically adsorbed rather than covalently linked. This result is paramount for biosensors that use NHS chemistry for protein immobilization due to effects that may arise from noncovalently linked proteins.

Facile synthesis of enzyme-inorganic hybrid nanoflowers and its application as a colorimetric platform for visual detection of hydrogen peroxide and phenol

This study reports a facile approach for the synthesis of horseradish peroxidise (HRP)-inorganic hybrid nanoflowers by self-assembly of HRP and copper phosphate (Cu3(PO4)2·3H2O) in aqueous solution. Several reaction parameters that affect the formation of the hybrid nanoflowers were investigated and a hierarchical flowerlike spherical structure with hundreds of nanopetals was obtained under the optimum synthetic conditions. The enzymatic activity of HRP embedded in hybrid naonflowers was evaluated based on the principle of HRP catalyzing the oxidation of o-phenylenediamine (OPD) in the presence of hydrogen peroxide (H2O2). The results showed that 506% enhancement of enzymatic activity in the hybrid nanoflowers could be achieved compared with the free HRP in solution. Taking advantages of the structural feature with catalytic property, a nanoflower-based colorimetric platform was newly designed and applied for fast and sensitive visual detection of H2O2 and phenol. The limits of detection (LODs) for H2O2 and phenol were as low as 0.5 μM and 1.0 μM by the naked-eye visualization, which meet the requirements of detection of both analytes in clinical diagnosis and environmental water. The proposed method has been successfully applied to the analysis of low-level H2O2 in spiked human serum and phenol in sewage, respectively. The recoveries for all the determinations were higher than 92.6%. In addition, the hybrid nanoflowers exhibited excellent reusability and reproducibility in cycle analysis. These primary results demonstrate that the hybrid nanoflowers have a great potential for applications in biomedical and environmental chemistry.

Direct electrochemistry of cytochrome c immobilized on a graphene oxide–carbon nanotube composite for picomolar detection of hydrogen peroxide

A new bionanocomposite electrode based on cytochrome c immobilized graphene oxide-multiwalled carbon nanotube is used to fabricate a highly selective and sensitive amperometric biosensor for the picomolar level detection of hydrogen peroxide which may find application in bioimaging and healthcare

Iron oxide magnetic nanoparticles with versatile surface functions based on dopamine anchors

The synthesis of multifunctional magnetic nanoparticles (MF-MPs) is one of the most active research areas in advanced materials as their multifunctional surfaces allow conjugation of biological and chemical molecules, thus making it possible to achieve target-specific diagnostic in parallel to therapeutics. We report here a simple strategy to integrate in a one-step reaction several reactive sites onto the particles. The preparation of MF-MPs is based on their simultaneous modification with differently functionalized dopamine derivatives using simple solution chemistry. The formed MF-MPs show comparable magnetic properties to those of naked nanoparticles with almost unaltered particle size of around 25 nm. The different termini, amine, azide and maleimide functions, enable further functionalization of MF-MPs by the grafting-on approach. Michael addition, Cu(i) catalyzed « click » chemistry and amidation reactions are performed on the MF-MPs integrating subsequently 6-(ferrocenyl)-hexanethiol, horseradish peroxidase (HRP) and mannose.

Inhibiting protein biofouling using graphene oxide in droplet-based microfluidic microsystems

Biofouling or adsorption of biomolecules onto surfaces in microfluidic devices limits the type of samples which can be handled. In this paper, we take advantage of the high adsorption capacity of graphene oxide (GO) for proteins as a strategy to limit biofouling, while preserving their activity for droplet-based lab-on-chip applications.