The growing interest in food quality and safety requires the development of sensitive and reliable methods of analysis as well as technology for freshness preservation and food quality. This review describes the status of chemical and biological sensors for food monitoring and smart packaging. Sensing designs and their analytical features for measuring freshness markers, allergens, pathogens, adulterants and toxicants are discussed with example of applications. Their potential implementation in smart packaging could facilitate food-status monitoring, reduce food waste, extend shelf-life, and improve overall food quality. However, most sensors are still in the development stage and need significant work before implementation in real-world applications. Issues like sensitivity, selectivity, robustness, and safety of the sensing materials due to potential contact or migration in food need to be established. The current development status of these technologies, along with a discussion of the challenges and opportunities for future research, are discussed.

Chemical and Biological Sensors for Food-Quality Monitoring and Smart Packaging.

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Solar light harvesting with multinary metal chalcogenide nanocrystals

Over the past ten years, considerable progress has been achieved in the field of nanomaterials-based enzymes (nanozymes). In comparison with natural enzymes, nanozymes demonstrate significant advantages such as facile synthesis procedure, low price, long storage period, and high environmental stability. A variety of nanomaterials including nanocarbons, metals, metal oxides, metal chalcogenides, halogen compounds, metal-organic frameworks (MOFs), and layered double hydroxides (LDHs) have been extensively investigated for enzyme mimicking. In this review, the recent progresses made in the development of the enzymatic properties of these nanozymes have been discussed. We comprehensively discuss strategies to improve catalytic activity and substrate specificity, enzyme-like catalytic mechanism, and novel application of nanozymes in sensing techniques. In addition, the remaining challenges and some future directions have been addressed. With the fast development of nanozyme applications in bioscience and technology, research in this field has become more and more attractive, which is expected to be a long-term exciting subject in the near future.

Functional nanomaterials with unique enzyme-like characteristics for sensing applications

Copper nanoclusters (Cu NCs) have emerged as a valuable member of the family of ligand-protected few-atomic metal nanoparticles and show fascinating properties of color-controlled light emission, combined with the advantages of versatile solution-based chemical synthesis at low cost. Synthetic methods of Cu NCs using various types of functional ligands and scaffolds allow tuning their emission wavelength and improving their environmental stability. Depending on the method of preparation and the ligands used, Cu NCs have already been applied for a wide variety of applications in catalysis, sensing, bioimaging, theranostics, and optoelectronics. This review highlights the potential of Cu NCs and links synthetic procedures and functionalization with different ligands with their properties and applications.

https://pubs.rsc.org/en/content/articlepdf/2019/qm/c9qm00492k

Ligand functionalized copper nanoclusters for versatile applications in catalysis, sensing, bioimaging, and optoelectronics

A colorimetric paper sensor for citrate as biomarker for early stage detection of prostate cancer based on peroxidase-like activity of cysteine-capped gold nanoclusters.

A facile strategy for detecting xanthine in serum samples by copper nanocluster (CuNCs) with high intrinsic peroxidase-like activity was reported. Firstly, a simple, mild and time-saving method for preparing CuNCs was developed, in which dithiothreitol (DTT) and bovine serum albumin (BSA) were used as reductant and stabilizer, respectively. The as-prepared CuNCs exhibited a fluorescence emission at 590 nm with a quantum yield (QY) of approximately 5.29%, the fluorescence intensity of the as-prepared CuNCs exhibited no considerable change when stored under ambient condition with the lifetime is 1.75 μs. Moreover, the as-prepared CuNCs exhibited high intrinsic peroxidase-like activity with lower K
 m
 (K
 m
  = 8.90 × 10−6 mol L−1) for H2O2, which indicated that CuNCs have a higher affinity for H2O2. Compared with natural enzyme, the as-synthesized CuNCs are more catalytic stable over a wide range of pH (4.0~13.0) and temperature (4~80 °C). Finally, an indirect method for sensing xanthine was established because xanthine oxidase can catalyse the oxidation of xanthine to produce H2O2. Xanthine could be detected as low as 3.8 × 10−7 mol L−1 with a linear range from 5.0 × 10−7 to 1.0 × 10−4 mol L−1. These results proved that the proposed method is sensitive and accurate and could be successfully applied to the determination of xanthine in the serum sample with satisfaction.

A novel colorimetric method based on copper nanoclusters with intrinsic peroxidase-like for detecting xanthine in serum samples

Aggregation-induced emission properties of hydrothermally synthesized Cu-In-S quantum dots.

Millimeter-sized α-glycine crystals were generated from continuous non-seeded cooling crystallization in slug flow. The crystallization process is composed of three steps in sequence: slug formation, crash-cooling nucleation, and growth. Stable uniform slugs of three different aspect ratios (slug length/tubing inner diameter) were formed, by adjusting the flow rates of both the solution and air streams. Besides supersaturation, the slug aspect ratio can also affect primary nucleation outcome. Stable slug flow can accommodate a relative supersaturation (C/C*) of up to 1.5 without secondary nucleation. Large glycine crystals can grow to millimeter size within 10 min, inside millimeter-sized slugs without reducing the slug quality.

https://www.mdpi.com/2073-4352/9/8/412/pdf

Continuous Generation of Millimeter-Sized Glycine Crystals in Non-Seeded Millifluidic Slug Flow

Glycine has been widely used as pharmaceutical excipients and synthesis reagents, and commercial glycine has a significant amount of aggregation and wide particle size distribution. A simple but reproducible process for generating uniform glycine crystals is always desired for both product quality and process efficiency purposes. Continuous cooling crystallization of glycine has been carried out in air-liquid slug flow in millimeter ID tubing, starting from solution without using seeds. Slugs were formed by combining air and liquid streams, then went through the crash cooling zone of varying lengths (tubing length in contact with ice bags). The operational boundaries of crash cooling times were evaluated: natural cooling (lower bound, no crash cooling), maximum cooling time for pure α-form without aggregation (upper bound), and beyond upper bound. Non-aggregating pure α-form glycine crystals were continuously generated within ~ 10 min, feasible from multiple conditions (combinations of crashing cooling time and starting concentration). When crash cooling time further increases (while maintaining the starting concentration), crystal aggregations and/or γ-form crystals could appear. Reducing starting concentration can allow longer crash cooling time without widening product crystal size distribution or reducing crystalline form purity. At proper conditions, even natural cooling in slugs can nucleate and grow non-aggregated pure α-form crystals. All cooling conditions carried out in slug flow generally minimize needle-shaped crystals compared with corresponding batches. Glycine crystals of α-form and narrow size distribution can be continuously generated within 10 min from cooling crystallization in millimeter-sized slug flow, without using external seeds nor adding solvent/additives. And, the operational boundaries of crash cooling time (at proper starting concentrations) for pure α-form non-aggregating product crystals are identified.

Fast Continuous Non-Seeded Cooling Crystallization of Glycine in Slug Flow: Pure α-Form Crystals with Narrow Size Distribution

A novel self-assembly phenomenon triggered by Zn2+ of Cu-In-S quantum dots with aggregation-induced emission effect was presented in this paper. Hydrophilic Cu-In-S quantum dots with aggregation-induced emission effect were successfully prepared. They were monodisperse spherical nanoparticles with the diameter of 2.8 ± 0.4 nm and had weak fluorescence in aqueous solution. However, the solution emitted strong fluorescence after addition of Zn2+. The results of TEM and SEM indicated the monodisperse quantum dots self-assembled into 3D networks of Cu-In-S quantum dots-Zn2+, which hindered the motion of quantum dots. Besides, the Zn2+ in the mixture passivated the surface defects. The phenomenon also proved by florescence lifetime and XPS. Thus the radiation decay decreased and followed by strong fluorescent emission. Interestingly, the degree of aggregation was proportional to the amount of Zn2+ and the fluorescent intensity. Based on this interesting phenomenon, a novel metal enhanced fluorescent nanosensor for detecting Zn2+ was established. The results demonstrated the proposed method had a good selectivity and linearity in the concentration range of 0–800 nmol L−1 with a limit of detection of 1.99 ppb. These results showed a promising future in the field of metal-enhanced fluorescent probes of the Cu-In-S quantum dots.

The self-assembly of Cu-In-S quantum dots with aggregation-induced emission into 3D network triggered by cation and its application as a novel metal-enhanced fluorescent nanosensor for detecting Zn (II)

Mingyao Mou

Papers

A novel colorimetric method based on copper nanoclusters with intrinsic peroxidase-like for detecting xanthine in serum samples

Aggregation-induced emission properties of hydrothermally synthesized Cu-In-S quantum dots.

Continuous Generation of Millimeter-Sized Glycine Crystals in Non-Seeded Millifluidic Slug Flow

Fast Continuous Non-Seeded Cooling Crystallization of Glycine in Slug Flow: Pure α-Form Crystals with Narrow Size Distribution

The self-assembly of Cu-In-S quantum dots with aggregation-induced emission into 3D network triggered by cation and its application as a novel metal-enhanced fluorescent nanosensor for detecting Zn (II)