Emerging Nanotechnologies for Water Treatment

Yanbiao Liu, Chong-Chen Wang, Wen Liu, Yanbiao Liu, Chong-Chen Wang, Wen Liu

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781839165108
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Scienze ambientali

Rapid population growth, urbanisation and industrialisation have caused serious problems in terms of water pollution and the supply of safe water. Solutions for monitoring pollutants in water and for removing them are urgently needed and they must be both efficient and sustainable.

Recent advances in emerging environmental nanotechnologies provide promising solutions for these issues. The physical and chemical properties of nanomaterials can be tailored by controlling attributes such as their size, shape, composition, and surface, so that they can be both highly specific and highly efficient. This makes them perfect platforms for a variety of environmental applications including sensing, treatment and remediation.

Providing an array of cutting-edge nanotechnology research in water applications, including sensing, treatment, and remediation, as well as a discussion of progress in the rational design and engineering of nanomaterials for environmental applications, this book is a valuable reference for researchers working in applications for nanotechnology, environmental chemistry and environmental engineering as well as those working in the water treatment industry.

Chapter 1

Functionalized Metal Nanoclusters for Biosensing Applications

Komal Kumari^1a, Debkumar Bera^1,b, Vinay Kumar^a, Surajit Rakshit^a and Nirmal Goswami,^{b, c}

^a Department of Chemistry, Institute of Science, Banaras Hindu University Varanasi 221005 Uttar Pradesh India;

^b Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Acharya Vihar, Bhubaneswar-751013, India;

^c Academy of Scientific & Innovative Research (AcSIR), CSIR – Human Resource Development Centre, Ghaziabad 201 002, India

*Emails: [email protected]; [email protected]

1.1 Introduction

Nanoscale materials are the foundation of many diagnostic technologies.¹ The unique physical and chemical properties of nanomaterials are often not possible to achieve in their corresponding bulk state and therefore, such nanoscale properties of these nanomaterials have been explored in many biomedical applications.^2–4 For instance, the localized surface plasmon resonance (LSPR) and electrochemical properties of noble metal nanoparticles (NPs) such as gold (Au) and silver (Ag) have been explored in sensing, diagnosis and therapy.^5–8 For biomedical applications such as sensing and imaging, the material should be highly photostable, nontoxic and optically sound. In this context, fluorescent organic dyes remained as one of the front runners for many years however, their poor photo-stability renders researchers to consider potential alternatives that will not only have extremely high fluorescence properties like organic dyes but also have good photo-stability in solution as well as in the solid state. Fluorescent nanomaterials such as semiconductor quantum dots (QDs) have been explored as one such alternative in the last few decades.^9–11 These QDs are extremely photostable, exhibit color-tunable fluorescence and have high fluorescence quantum yield. Though one key issue of exploring these QDs in biomedical application is their intrinsic toxicity which originates due to the presence of toxic metals such as Pb²⁺/Cd²⁺/Hg²⁺ as key constituents. Indeed, several functionalization strategies have been developed to reduce the toxicity of QDs however, none of them appears to provide a long-term solution.^12,13

Consequently, the focus of research was shifted to the development of new non-toxic nanomaterials with controlled fluorescence properties. In this context, metal nanoclusters (MNCs) have emerged as a rising star in the fluorescent nanomaterial community.^14,15 They consist of a few to hundreds of metal atoms at the core with an average size less than 2 nm. Because of their ultra-small size, these MNCs exhibit many unique physical and chemical properties such as HOMO–LUMO transition, fluorescence, chirality, intrinsic magnetism, high catalytic activity, etc.^16–21 Because of such molecular-like properties, which are not generally seen in their larger counterparts (i.e., nanoparticles; size 3–100 nm), MNCs are considered to bridge the gap between atoms and nanoparticles. All these intriguing molecular-like properties allow MNCs to be used in various areas including optoelectronics, photovoltaics, catalysis, sensing, imaging and so on.^22–28

In the last two decades, research on MNCs has advanced tremendously. In particular, various synthesis methodologies have been developed to achieve MNCs with high atomic precision, functionalization strategies have advanced to improve the properties of MNCs and subsequently, their properties have been explored in various avenues. Because of such advancements in the field, several review articles were published over the last few years. These review articles focused on the total synthesis, synthesis mechanism,²⁹ optical and fluorescence properties of MNCs,^26,30 sensing,²⁵ catalysis²² and biomedical applications of MNCs.^31,32 Some excellent review articles are also well documented that summarize a brief overview of the nanocluster field and their potential in various applications.^14,15,23,26 However, a concise review article that is particularly focused on the biosensing applications of MNCs is missing in the literature. Biosensing is an important research field, with numerous applications in the health and environment sectors. Over the last few years, several published articles suggested that MNCs could be the ideal biosensor material due to their excellent biocompatibility and optical properties. They have been used to detect biologically relevant small molecules (e.g., H₂S, H₂O₂, cysteine, glutathione, dopamine and so on) and biomolecules (e.g., proteins, DNA, RNA, ATP and so on).^33–40 Therefore, a focused review on the development of functional MNC-based biosensors would be of great interest. In this chapter, we summarize the state-of-the-art biosensing applications of MNCs. In particular, this chapter is focused on sensors based on the absorbance, fluorescence and electrochemical properties of MNCs. A brief discussion on the current challenges and an outlook on the future research challenges of these functional MNCs are also presented.

1.2 MNC-based Optical Biosensors

Biosensors relying on the optical properties of materials have certain advantages over other prevailing methods, such as high sensitivity, rapid detection and low cost of spectroscopic assays. Being ultra-small in size, MNCs not only show distinct absorbance bands in their UV–vis spectra but also exhibit size- and composition-dependent tunable fluorescence properties that are highly sensitive to the environment. In this section, we summarize recent advances in the application of MNCs as new fluorescent probes for analytical sensors and biological imaging.

1.2.1 Detection of Small Biomolecules

Amino acids such as cysteine (Cys), lysine (Lys), histidine (His), etc. play a vital role in biological systems. The lack of any of these amino acids or the excess presence of them could lead to severe diseases and therefore the selective detection of these amino acids is of great importance in biomedical research. As an example, elevated levels of Cys are associated with diseases like hypoglycemic brain damage, schizophrenia, etc.,^41,42 while low levels of Cys are involved in liver injury, skin damage, and weakness or even Huntington's disease.⁴³ With the aim of detecting Cys, various MNCs and their composites have been developed. Trace level of Cys can enhance the photoluminescence (PL) properties of MNCs through the passivation of their defect states. Based on this mechanism, red-emitting bovine serum albumin protected AuNCs have been used to detect trace level of Cys in human blood serum. Recently, an aggregation-induced emission (AIE)-type AuNCs-based Cys sensor has also been constructed under alkaline conditions (pH 11). At elevated pH, the PL intensity of the AIE-type AuNCs is generally enhanced. When Cys was added at an alkaline pH, the PL of these NCs was significantly quenched. By this method, it was not only possible to achieve an ultra-low limit of detection (LOD) but also high selectivity over another ten amino acids and glutathione (GSH).⁴⁴ Such a high selectivity for Cys and homocysteine (Hcy) was also observed when AuNCs–NBD (4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl)) was used as a chemosensor.⁴⁵ Due to the high specificity of NBD toward Cys and Hcy, the AuNCs–NBD probe was successfully applied for monitoring and imaging intracellular Cys and Hcy in HeLa cells. Recently, a Förster resonance energy transfer (FRET) assembly using GSH capped AuNCs and carbon dots as a ratiometric probe for cysteine detection was developed, which may be employed in the future for the diagnosis of cysteine-related diseases.⁴⁶ Like Cys, fluorescent AuNCs were also used to detect other important amino acids like histidine or lysine in biological fluids. For histidine detection, the PL of the NCs was often quenched by bivalent metal ions such as Cu²⁺ or Ni²⁺. Due to the strong chelating efficiency, upon exposure of free histidine, these metal ions could be removed from NC surface and as a result, the PL of the AuNCs was restored.^47,48 A ratiometric fluorescent sensor based on GSH-stabilized AIE-type AuNCs was also reported for the detection and sensing of lysine and Cys.⁴⁹

A FRET-based sensing platform was also employed for the detection of hydrogen sulfide (H₂S) in both in vitro and in vivo systems using chitosan-functionalized AuNCs.³³ Hydrogen peroxide (H₂O₂), which is a key messenger in redox signaling, enzyme activity and various cell signaling pathways, is another important analyte often detected using fluorescent MNCs.³⁴ In particular, various protein- (or enzyme) functionalized MNCs were frequently used to detect H₂O₂. Upon addition of H₂O₂, the PL intensity of these protein-protected MNCs was quenched primarily due to the surface oxidation followed by aggregation. Such quenching strategies were used for intercellular H₂O₂ sensing and imaging of living macrophage cells using BSA-protected AuNCs.³⁵

In addition to the functionalized AuNCs, other MNCs such as AgNCs with strong PL properties could be regarded as effective biosensors. For example, Lan et al. prepared highly fluorescent AgNCs b...