Nanomaterials for analytical chemistry

Over the last few decades, nanomaterials have been playing increasingly important roles in developing analytical biosensors.With a large specific surface area, nanomaterials offer various surfaces for immobilization of biological probes.In addition,taking advantage of their magnetic, catalytic and various optical properties such as fluorescence emission and quenching, light absorption and scattering,Raman enhancement,localized surface plasmon resonance, versatile signaling and signal amplification methods can be realized. Many types of biomolecules, such as antibodies,aptamers and nucleic acid probes have been combined with nanomaterials.Using nanomaterials to mimic the binding and catalytic activities has produced molecularly imprinted polymers and nanozymes.These bio/nano molecular recognition systems are fundamentally interesting and practically useful.This special issue includes nine reviews and ten original research papers,highlighting some of the recent research progress in nanomaterial-related analytical chemistry.

Many nanomaterials possess unique physicochemical properties that are difficult to realize by small molecules. Yuan and coworkers reviewed recent advances on using persistent luminescence nanoparticles to reduce background fluorescence and achieve autofluorescence-free biosensing and bioimaging applications. The emission of these nanomaterials can last a long time after stopping the excitation (https://doi.org/10.1016/j.cclet.2019.06.016). Lou, Xia and coworkers reviewed efforts towards detection of multiple target molecules in complex biological samples using one and two dimensional nanomaterials to decrease false positive results (https://doi.org/10.1016/j.cclet.2019.06.025).Analysis of chiral molecules is a very important yet challenging task. Li and coworkers reviewed enantioselective analysis based on noble metal and semiconductor nanomaterials with various optical signaling methods including fluorescence,colorimetry and circular dichroism (https://doi.org/10.1016/j.cclet.2019.05.036).

By rational designed nanomaterials, many interesting biosensors can be produced. Raltitrexed is an anticancer drug. Qi and coworkers reported a hybrid nanomaterial containing gold nanoparticles as fluorescence quenchers and [6_TD$DIF]D-proline capped gold nanoclusters as fluorophores. In the presence of Raltitrexed, the fluorescence can be recovered allowing sensitive detection(https://doi.org/10.1016/j.cclet.2019.05.019). Tang and coworkers reported a signal amplification immunoassay system containing magnetic beads, DNA-functionalized gold nanoparticles and two glucose oxidase labeled hairpin DNA strands. The reaction produces a large amount of surface immobilized glucose oxidase that can convert glucose to gluconic acid,allowing monitoring the reaction with a simple pH meter (https://doi.org/10.1016/j.cclet.2019.03.045).

Liu et al.took advantage of the literature database and rationally designed fluorescent carbon dots with a pKavalue of 6.84,allowing highly sensitive measurement of intracellular pH(https://doi.org/10.1016/j.cclet.2019.06.012). Lv et al. synthesized fluorescent carbon dots that can be quenched by either acid or by hydrogen peroxide in the presence of iodide.Acid and H2O2are the oxidation products of glucose by glucose oxidase. Taking advantage of this carbon dot, highly sensitive detection of glucose was achieved(https://doi.org/10.1016/j.cclet.2019.06.029). Cao and coworkers prepared size-controllable graphitic carbon nitride quantum dots(g-C3N4QDs), whose emission was selective quenched by Cu2+.They demonstrated a highly sensitive copper sensor in their work(https://doi.org/10.1016/j.cclet.2019.05.058).Song et al.modified a carbon fiber microelectrode with gold nanoflowers and overoxidized polypyrrole, allowing highly sensitive detection of serotonin, one of the monoamine neurotransmitters (https://doi.org/10.1016/j.cclet.2019.05.042).

Developing enzyme mimics has intrigued chemists for many decades.From the bioanalytical perspective,enzyme mimics may replace protein-based enzymes for both molecular recognition and signal amplification purposes. This special issue has three papers describing dehydrogenase, peroxidase and oxidase mimicking activities,respectively.Liu and coworkers reported that small gold nanoparticles have dehydrogenase-like activity and can help convert estradiol into an oxidation product. This is a new activity of gold nanoparticles and an interesting example for environmental applications (https://doi.org/10.1016/j.cclet.2019.05.062).Zhang and coworkers found that a polyguanine DNA oligonucleotide has peroxidase-like activity when mixed with Cu2+. They designed an immunoassay with antibody-labeled CuO nanoparticles, which after dissolution by acid can produce the needed Cu2+(https://doi.org/10.1016/j.cclet.2019.05.037). Deng et al. explored chitosan-stabilized platinum nanoparticles for their oxidase-like activity to convert TMB into blue colored product. The activity was weakened by Ag+ions allowing sensitive and selective detection of silver (https://doi.org/10.1016/j.cclet.2019.05.032).

In addition to applied research,a few fundamental studies were also included.Although most bioanalytical work used unmodified DNA probes,over 150 types of chemical modifications are known in nucleic acids. Yuan and coworkers reviewed various mass spectrometry, sequencing, and microscopy based analytical methods with the help of enzymes and nanomaterials for identification of such modifications (https://doi.org/10.1016/j.cclet.2019.02.005).Gold nanorods are very popular for their color and Raman enhancement properties. Xia and coworkers used H2O2-based etching system without or with Fe2+to study the effect of the diameter of gold nanorods on the analytical performance as colorimetric biosensors. They concluded that thinner nanorods possess a higher sensitivity (https://doi.org/10.1016/j.cclet.2019.06.038).

Another suite of papers describe nanomaterial enhanced detection. Qiu and coworkers reviewed the use of nanomaterials to enhance fluorescence polarization/anisotropy based sensors.Since nanomaterials have a very high molecular weight, they can significantly increase fluorescence polarization upon adsorption of fluorophores (https://doi.org/10.1016/j.cclet.2019.06.005). Chen and coworkers summarized an interesting type of hybrid nanomaterial containing a metal nanoparticle core and a graphitic shell.The shell can quench fluorescence and block reactions catalyzed by the metal core. The shell has characteristic Raman bands also useful in SERS biodetection and bioimaging (https://doi.org/10.1016/j.cclet.2019.05.049). Electrochemiluminescence (ECL) is now a key analytical technique in commercialized bioassays, and developing nanomaterial based new emitters has attracted many interests. Su and coworkers reviewed several new and representative applications of ECL imaging using and micro-/nanostructured nanomaterials (https://doi.org/10.1016/j.cclet.2019.05.038).Shi et al. introduced various sensors and bioassays using nanomaterial-based ECL systems (https://doi.org/10.1016/j.cclet.2019.04.066).Finally,while most of the above papers focused on nanomaterials, Gu and coworkers reviewed solid-state nanopore-based analytical systems for ions and small molecules(https://doi.org/10.1016/j.cclet.2019.06.011).

These papers have touched many important analytical applications of nanomaterials, from molecular recognition, producing signals to signal amplification. This relatively small collection of papers are unable to capture all the important advances. We focused on sensing, while some other applications such as separation and extraction were not included.We hope this special issue can be useful for readers to get a big picture on the field from the selected examples.

We are grateful to all the authors for contributing high quality manuscripts, referees for their constructive comments, and the Editorial Office,especially Editor Fenglian Jiang and Dr.Huanfang Guo (Associate Editor-in-Chief) for their support along the way.