NSs had a very good 100% degrading capability of Rh B in visible region, where h+ of VB (Bi2MoO6) and e− of CB (CNQDs) worked effectively for oxidation/reduction to cause degrading reaction to give H2O and CO2 [171]. Si NWs (silicon nano-wires) on g-C3N4 QDs as Si NWs @ g-CNQDs, photoelectrocatalytically could decompose 85.1% of 4-CP in ~ 2 h from aqueous solution, had a notable charge separation and good stability [172]. π-conjugated GCNQDs implanted on metalloid sulfide Sb2S3/supported by ultrathin-g-C3N4, with a bandgap of 2.7 eV, were proved to be good candidate for photocatalytic disposals of MO from unwanted water and had a very good electron (e−) transference [173]. In a novel approach, the co-workers Patel, J. et al. synthesized Mn:ZnS/QDs, for photo-degrading fluoroquinolone: Norfloxacin in an ambient condition of solar-light/UV-light, where Mn and ·OH fortified the reaction to 4 reapplied cycles [174].
1.5.2 Polymer Composite–Based FNMs as Photocatalysts
Polymer TiO2/CS/glass FNMs were powerful in decomposing RR4 organic dye in visible region. h+ and ·OH generated from TiO2 layer circulate to TiO2/CS boundary to cause oxidation of RR4. A total of 100% efficiency was noticed with the stability up to seven reusable cycles [175]. CdS/TiO2-PAN FNM degraded MB (66.29%) in 210 min [176]. Researchers studied the photocatalytic action and inferred a repeated utility to protect the water system. Chitosan-AgCl/Ag/TiO2 synthesized by the team Jbeli, A. et al. was reported to be cost-effective photocatalytic degrader of organic components ABA, O-TD, and SA under visible radiations [177]. Similarly, surface modified FNM TiO2/ZnO/chitosan had a powerful photo-degradability of MO (97%) when excited by solar radiations [178]. An organic/inorganic FNM as composites P3HT/PNP-Au NP got by re-precipitation method showed positive spectral line in UV region (~427 nm), had an enhanced photocatalytic decomposition of MB (90.6%), and inferred that it may be due to a strong π-π* shift [179]. A 3D honey-comb like ordered macro-porous NM-3DOM Ag/ZrO2 had significant photocatalytic degradability over CR when stimulated by multi-modules of microwave-assisted, simulated-solar, UV, and visible radiation [180].
1.5.3 Metal/Metal Oxide-Based FNMs as Photocatalysts
Metal/Metal oxide when entrapped as FNMs, on photo-irradiation leads to photo-excitation of particles that undergoes transference between the CB and VB, where the CB transfers the e− to degrade the pollutant [181]. Components like (1) semiconductor, (2) metal, and (3) metal-supporters assist interfacial-interaction for photocatalytic degradative actions. In one of their reports, Park, S.J. et al., proved that T-ZnO-CNO FNMs with nano-onions prepared by green routes removed the challenging pollutant of water 2,4-DNP photocatalytically with an efficacy of ~92%. Conversion of O2 to ·O2− and formation of ·OH favored the degradation using 3D hybrid structures [182]. Degradation of phenol (63%) and MB (52%) at 420 nm (visible) by FNMs of Au/(WO3/TiO2) and WO3/TiO2 were noticed by Rhaman, Md. et al. [183]. Electron transference is retarded but hole movement is favored due to Au embedded on the surface of WO3/TiO2. Sufficient bandgap energies cause photocatalytic excitations. Flower-like functionalized Au-ZnO NMs hydrothermally were responsible contributors for photo-decomposition of Rh B into CO2 and H2O, with h+(holes) and ·OH formed by light radiations were functional for the activity. A total of 99% reduction in 10 min was noticed by authors Hussain, M. et al. [184].
1.6 Nanocatalyst Antimicrobials as FNMs
FNMs as nanocatalyst have been authenticated with a promising note for cleansing and sanitization treatment for a mixture of waste and normal water from different sources. With an excellent potentiality to inactivate the active disease-causing dreadful pathogenic micro-organisms like fungi, bacteria, and viruses, FNMs behold their role to safeguard the water bodies. Increase in the potential momentum for anti-microbial activity is efficaciously observed by surface alterations of NPs [211]. Functionalized photocatalytic materials are appropriate tools for curbing the unwanted horrible [212]. Disinfection rate of pseudo-1st-order rate kinetics for retarding activation of E. coli was observed to be 86% in 4 h by FNMs (C60/C3N4 | C70/C3N4) on photocatalytic treatment [213]. Similarly, 2D g-C3N4 NS/PAN hydrophilic filter membranes, demonstrated for excellent self-cleaning and anti-bactericidal action over E. coli, by photocatalytic visible light were proven to have 100% sterilization efficiency at optimized (12 h) condition with a restoration capacity to about three rescalable cycles [214]. The existence of the hindering polymeric substances on pathogens that retard the anti-microbial photocatalytic efficacy is to be negated for water disinfections, in a momentous scale [215].
Visible light illuminations are powerful in inhibiting the bactericidal action by the photo-generated peroxides from g-C3N4 NS [216]. In an anti-bacterial work, the co-workers defied the potent E. coli, by Ag/TiO2 film, using photocatalytic fluorescent light radiation for 3 h. A total of 81% deactivation was observed, due to the release of Ag+ ion, and the trapped photo-induced e−, h+, that diffuse on the catalytic surface to inhibit the activity of the target species [217]. Hence, the cell wall membrane of bacteria is disrupted by the photo-generated e− leading to reductive cell damage thereby deactivating the bacterial activity. gC3N4-BiFeO3-Cu2O FMNs by photocatalytic decomposition using visible light suppressed the activity of the bacterial stains, S. aureus (G+) and E. coli (G−) in ambient reaction condition [218]. In some reverse trials, the bacterial E. coli affecting the water bodies when functionalized with TiO2 as cocktails promoted the deactivation of the bacterial E. coli and S. epidermidis [219]. Abundant trials have made their way to upsurge the usefulness of FNMs as nanocatalyst, as anti-microbials by optimized advancement with molecular modifications with resourceful materials.
1.7 Conclusions and Future Perspectives
In this existing scenario, presently, the whole world is confronting a critical water challenge. With a greater reason, persevering and safeguarding the prevailing fresh natural water reserves and new water segments is an immediate need, to overcome this global challenge faced. Reports of WHO (water hygiene and sanitation) infer that 1,870,998 people (WHO 2019), especially below the age group of 5 [(361,000 deaths) (WHO 2014)], die every year, due to unsafe water supplies, caused by contaminations in aquatics [220].
In a broader perspective, nanotechnology affords better promises to solve the existing crisis, ensuring a good quality potable/consumable water by eliminating the undesirable harmful biological and chemical contaminants from water bodies. The salient features that make the NMs occupy a highest platform in the environmental remediation methodologies are (1) a greater ratio of surface:volume, (2) high stability and reactivity, (3) assorted morphological shapes and sizes, and (4) good reusability. Henceforth, NMs find their way to detect and eliminate the unmanageable contaminants (toxic gases, noxious heavy metals, toxin organics, and undesirable biological components) form the water bodies in an effectual way [221].
Similarly, the risks to human health caused by contaminants in food and water (pesticides, metallic debris, industrial wastes, dyes, and drugs) can be well curtailed, when detected with the help of NMs. NMs (FNMs: QDs, CQDS, CNQDs, MWCNTs, and MNPs) as sensors or biosensors, are found to be self-reliant, efficient, and cost-effective. The noteworthy exceptional thermal and chemical, mechanical, and electro-optical features render NMs to function as a trouble-free, inexpensive, and rapid tool in detecting the toxins present in food and water [222].
Analogously, various environmental methodologies (nano-photo catalysis, membrane
