The economical and feasible design of nanomaterial catalysts for cutting-edge applications, as well as environmentally friendly catalytic processes and sustainable methods for developing synthetic strategies for catalysts, have received much attention from researchers in recent years. In this respect, scientific research is continually enhancing and enabling the synthesis of novel materials and applications [1]. Noble metal nanoparticles have a high degree of functionality due to their unique physical–chemical properties [2]. Their high stability, surface functionalization, and easy chemical synthesis makes the noble metallic nanoparticles, such as PtNPs, AuNPs, and AgNPs, extensively utilizable [3]. However, some earth-abundant and inexpensive metals have attracted attention in this regard over the expensive noble-metal catalysts that are used widely in conventional commercial chemical processes.

Owing to their high natural abundance, low cost, and numerous and practical simple syntheses, copper-based nanomaterials and nanocomposites are particularly appealing for research [4]. Due to their unique properties, copper nanoparticles are progressively becoming a key component in a variety of industries, such as energy, pharmaceutical, electronics, machinery, construction, engineering, environment, etc. In recent years, researchers focusing on sustainable approaches for environmentally friendly catalytic processes for various advanced applications.

2. Copper-Based Nanomaterials for Environmental Pollution Management

2.1. Copper-Based Nanomaterials in Photodegradation of Industrial Dyes/Removal of Dyes

Recent advances in the food, paper, leather, and textile industries have rapidly increased the usage of organic dyes. This high usage of dyes, which are the main sources of organic pollution in various industries, has led to a global concern ^[5][6][7][5,6,7]. Since these dyes are extremely persistent, and carcinogenic to humans and other living things, even very low quantities of chemical pollutants in wastewater cannot be eliminated easily by regular methods such as sedimentation and ordinary chemical degradation ^[5][8][5,8]. Hence, removal of these effluents is important. Therefore, a sustainable technique for the removal of such effluents is essential to remove these hazardous pollutants, and therefore efficient catalytic reduction and photocatalytic degradation are essential ^[9][10][11][9,10,11]. The two oxidation states of copper, Cu²⁺ and Cu⁺, act as electron trappers, leading to a higher degradation efficiency of copper nanoparticles. The catalytic property of CuNPs was observed in the reduction activity of Xanthene dye that could be applicable in biological sensing [12]. In a study, to treat textile wastewater, it was observed that the degradation activity of CuO NPs was higher than Ni@Fe₃O₄ against organic dyes such as Congo red, methylene blue, and Rhodamine B ^[13][87], and also showed reduction of 4-nitrophenol ^[14][13].

2.2. Copper in Reduction of Other Heavy Metal Contamination

In developing countries, the industrial sector is rapidly increasing, and the heavy harmful metals from these industries, such as metal and mining, batteries, paper, pesticides, chemical and petrochemical, textile, leather, cement, etc., are released into water bodies. These toxic heavy metals, such as lead (Pb), chromium (Cr), cadmium (Cd), arsenic (As), etc., do not break down further in the environment and accumulate into the vital organs of animals, causing various chronic diseases and death in extreme cases ^{[15][16][17][18][19]}[98,99,100,101,102]. A highly porous material, Cu-DPA MOF ^[20][103], was synthesized for the removal of heavy metals from wastewater. The adsorption parameters, such as pH value, contact time, initial metals concentration, and Cu-MOF dosage, exhibited significant adsorption processes in the removal of heavy metals such as Pb, Cd, and Cr. The effect of adsorbent dose, pH, metal ion concentration, contact time, and time of mixing to reach equilibrium for these heavy metals by Cu-DPA MOF was determined through batch adsorption experiments. An optimized procedure was performed in order to carry out this procedure on wastewater containing Cd, Cr, and Pb ^[20][103].

2.3. Copper-Based Nanomaterials in Wastewater Treatment

As a result of rapid industrialization, the volume of pollutants has increased. Industrial by-products often include dangerous and cancer-causing synthetic organic dyes, pesticides, pharmaceuticals, and textile waste. If these materials are not disposed of properly, they can have a detrimental effect on the environment. Manufacturing processes often use synthetic organic dyes which are highly stable and do not break down easily. As a result, the wastewater can be toxic to humans, animals, and plants, and it can contaminate surface- and groundwater ^[21][22][110,111]. This can lead to serious environmental issues due to the compounds’ ability to remain stable in the environment ^[23][26]. Many conventional methods such as photocatalytic degradation, coagulation, advanced oxidation processes, etc., have been used to remove pollutants from water and wastewater. These methods are less effective in meeting the stringent standards of water quality, and many emerging technologies have evolved ^[24][112]. Due to the pore size, the high surface area of the nanomaterials which has unique properties, such as photosensitivity, antimicrobial activity, catalytic activity, magnetic, electrochemical, and optical properties, provides a wide range of applications in the field of remediation of pollutants, detection, and water quality monitoring ^[25][26][113,114].

2.4. Copper-Based Materials as Biosensing Materials

Graphene oxide (GO) and copper oxide (CuO) nanocomposites were used to successfully create an enzyme-free amperometric glucose biosensor using a fluorine-doped tin oxide (FTO) substrate ^[27][61]. In the presence of phosphate-buffered solution at pH 7.0, glucose sensing was performed. It was shown that the prepared sensor exhibited excellent electrical conductivity, and a low detection limit in human serum in comparison, which is perhaps due to the large superficial area that leads to good catalytic activity. The investigation of DNA methylation, a crucial biological process, holds significance as it can alter the activity of DNA segments without modifying their sequence. Methylation of DNA has been observed to frequently impede gene transcription, especially when it takes place at a gene promoter. This understanding underscores the importance of studying DNA methylation and its impact on gene regulation. DNA methylation is crucial for proper growth in mammals and is also associated with aging, genomic imprinting, carcinogenesis, X-chrome inactivation, and so on. In a study, modified reduced graphene oxide (rGO) which was decorated on CuNPs was used as the framework for a label-free DNA-based electrochemical biosensor that might be employed as a diagnostic tool for a DNA methylation assay ^[28][60]. 

2.5. Copper-Based Nanomaterials in Pesticide Remediation in Soil

Despite being officially prohibited in many countries, several pesticides and insecticides are still in use today; for instance, endosulfan and carbofuran ^[29][127]. One of the most frequently used sulfur-containing organic molecules are dithiocarbamates, with major applications in the production of sugar, rubber manufacture, antioxidants, and antislime in paper making ^[30][31][128,129]. Between 25,000 and 35,000 metric tons are estimated to be consumed annually in this manner ^[32][33][130,131]. These dithiocarbamates are categorized into different classes, such as zineb, ferbam, maneb, etc., based on the carbon skeleton and properties ^[34][132].

2.6. Copper-Based Nanomaterials in the Degradation of Pharmaceutical Products

One of the three major sources of drugs and their metabolites entering the environment is the pharmaceutical industry, where they are released during the production of drugs. Poor waste management from industries, hospitals, and homes can also cause these substances to be discharged without being treated. Wastewater and sewage sludge from municipal wastewater treatment plants are considered a “dispersed” source, with drugs excreted by humans in homes, hospitals, and other health facilities entering these systems. The use of effluents and biosolids for fertilizing purposes also contributes to the release of pharmaceuticals into the environment ^[35][135]. One of the most used strategies to detoxify contaminants of emerging concern (CECs) is bioremediation. Bioremediation involves the use of microorganisms to break down the CEC molecules into harmless by-products. Similarly, another approach to detoxifying CECs is nanoremediation. Nanoremediation involves the use of nanomaterials, such as nanoparticles, to absorb and trap the CEC molecules. Nanoparticles can be engineered to specifically target CEC molecules and have been used in a variety of environments, including soils, sediments, and aquatic systems. Nanoremediation has been shown to be more effective than bioremediation in some cases, as it can target specific CEC molecules more effectively by using physico-chemical treatments that include adsorption, oxidation, and filtration. Adsorption involves using particles, such as activated carbon, to adsorb the CEC molecules from an environment. Oxidation involves using chemical oxidants, such as ozone, to break down the CEC molecules into harmless by-products.

2.7. Copper-Based Nanomaterials as VOC Sensors

VOC (Volatile Organic Compound) sensing is necessary in industries to ensure that the air quality is safe for both employees and on the roads to ensure safe driving of individuals, as alcohol intoxication is the primary cause of road accidents in the U.S. and worldwide ^[36][143]. By monitoring levels of VOCs, industries can ensure they are compliant with safety regulations and that their workers are not exposed to dangerous levels of these compounds. Prolonged exposure to VOCs can lead to respiratory problems and kidney damage; some VOCs have been linked to an increased risk of certain types of cancers, such as leukemia and lymphoma, and long-term exposure leads to headaches, dizziness, memory loss, and other neurological effects ^[37][144]. Some molecules, such as ammonia, hydrogen sulfide, hydrogen peroxide, etc., are considered VOC biomarkers and their detection plays a vital role. For instance, ammonia in the exhaled breath indicates several diseases, such as type II Alzheimer’s, kidney failure, hepatic encephalopathy, and liver dysfunction ^[38][145]. Copper has been used as a sensor for VOCs for over a decade and has proven to be a reliable and cost-effective method for detecting these compounds.

2.8. Copper-Based Nanomaterials in On-Site Sensing

On-site sensing strategies for the detection of environmental contaminants play a crucial role in environmental monitoring and pollution management. These strategies enable real-time or near real-time analysis of contaminants directly at the site of interest, providing valuable information for decision-making, remediation efforts, and ensuring the safety of ecosystems and human populations. The development of on-site sensing technologies has seen significant advancements in recent years, offering increased sensitivity, selectivity, portability, and ease of use. Recent technological advances and novel scientific developments have been made in fabricating efficient point-of-care technologies, portable devices, and miniaturized tools for the on-site detection of hazardous contaminants ^[39][40][41][152,153,154]. These emerging technological approaches make the screening of hazardous contaminants more strategic, highly efficient, executable, rapid to operate, and cost-effective. Moreover, the design and development of on-site sensing technologies has transformed the proficiency of environmental contaminant detection. On-site sensing assays range from minimal techniques to complex and advanced investigating tools and a plethora of recognition formats. Compared to conventional and traditional sensing strategies, the use of point-of-care technologies for the on-site detection of environmental contaminants shows significant advantages with respect to the integration, levels of automation, input consumption, miniaturization, reliability, turnaround velocity, size of device, precision, ease of use, and initial detection of environmental contaminants.

2.9. Copper-Based Nanomaterials in Carbon Dioxide Reduction

CO₂ electroreduction can transform intermittent energy sources into high-energy chemicals, reducing dependence on fossil fuels and pollution. Products like hydrocarbons and methanol, with high energy density, are compatible with existing infrastructures and can substitute for fossil fuels ^[42][157]. CuO-ZnO nanomaterials are employed as catalysts for the conversion of carbon dioxide into methanol. Converting carbon dioxide into methanol determined that bimetallic systems combined with porous supports, such as zeolite and activated carbon, had a greater efficiency when compared to unsupported materials. Hydrogenation at different temperatures was carried out in a stainless-steel-packed bed reactor for the conversion to methanol, which indeed reduced the environmental emissions of carbon dioxide emissions ^[43][158]. In a certain CO₂ reduction experiment, which was conducted electrochemically in two compartments of H-cell in ethylene production, copper oxide nanoparticles were used as the catalyst. The ethylene production was dependent on the morphology of the catalyst. The CuO nanoparticles were deposited on conductive carbon materials which were activated, and the copper species converted to Cu⁺, which eventually resulted in the formation of 70% ethylene and 30% of hydrogen Faradaic efficiency (FE) without any other by-products in an aqueous solution ^[44][159].

3. Conclusions

In conclusion, copper-based nanomaterials have shown great promise for environmental remediation such as wastewater treatment (dyes, pesticides, and heavy metal removal), biosensing, VOC sensors, and CO₂ reduction, etc. This extensive article summarizes many efficient methods proposed mainly to examine and assess recent publications on Cu-based materials and give an overall view and identify potential future work areas in the remediation of the environment. Even though many promising biological methods are found to be safer, Cu NPs are biocompatible and non-toxic, and are capable of removing hazardous metals from contaminated water, soil, and air, as well as breaking down toxic organic compounds, making them ideal for use in environmental remediation. However, for perfect environmental management, research should not be confined to detection, but potential future work and further investigations should be extended in different areas of synergetic effects in environmental management in the areas of reduction, degradation, reuse, recycling, etc.