Recent Advances in Copper-Based Materials for Environmental Remediation

1. Introduction

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 got a huge amount of 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 (NMNPS) have a high degree of functionality due to their unique physical-chemical properties [2]. The high stability, surface functionalization, and easy chemical synthesis makes the noble metallic nanoparticles, such as PtNPs, AuNPs, and AgNPs in their extensive utilization [3]. However, some earth-abundant and inexpensive metals have been attracted the attention in this regard over the expensive noble-metal catalysts that are used widely in conventional commercial chemical processes.

Owing to the high natural abundance, low cost, 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, construction, machinery, construction, engineering, environment, etc. In recent years, researchers focussing on sustainable approaches for environmentally friendly catalytic processes for various advanced applications. This review paper does not promote various chemical methods for synthesis, however, helps the future research for an overall idea about the various synthesis and various applications of copper nanomaterials and copper-based nano in a sustainable way.

Dennis et al. reviewed the use of nanomaterials and plant extracts for the removal of micropollutants from wastewater streams. Also analyze their efficacy in removing these contaminants, as well as their cost-effectiveness and sustainability [5]. Khalaj et al. reviewed the investigation of the toxicological, environmental, and operating effects of copper-based nanomaterials for the treatment of persistent effluents such as dyes and their effects on the nanomaterial’s reactivity [6]. Crisan et al. made a recent review paper that investigates the potential of copper nanoparticles as an alternative to antibiotics in the fight against multi-resistant bacteria strains [7]. Sandoval et al. reviewed the generation of copper-based particles aimed to discuss the pretreatment, milling, and post-treatment steps, as well as the characterization methods used to analyze the resulting particles [8]. Hong et al. investigated the effects of bimetallic catalysts on the selectivity of carbon dioxide reduction reaction (CO₂RR) and analyzed the effects of different ratios of metal atoms in the bimetallic catalysts on the CO₂RR of different ligands. [9] Naz et al. overviewed the synthesis, biomedical applications, and toxicological assessments of copper nanoparticles which aimed to provide core knowledge to researchers in this field, in order to conduct future studies [10]. Therefore, this present review article explores the potential of copper in various areas of environmental remediation such as the degradation of dyes, pharma products, wastewater treatment, and pesticides, and as a sensor for detecting various pollutants and reducing carbon dioxide (CO₂) emissions. Copper has been identified as an effective tool in these areas due to its high reactivity and low cost. The article reviews the recent research in these areas and highlights the potential of copper in each application. For example, the use of copper in dye degradation has been shown to be effective in reducing water pollution, while copper can also be used in wastewater treatment to reduce levels of heavy metals. The article also emphasizes the need for further research in order to fully understand the potential of copper in these applications.

The review paper also discusses potential challenges and future opportunities for the use of copper-based nanomaterials in environmental remediation.

2. Materials and Methodology

In comparison with other metals, Copper being less toxic, and inexpensive metal, and copper-based materials can be recycled and reused again [11]. Copper nanomaterial synthesis is simple but proper experimental conditions with respect to time are somewhat challenging in comparison. Copper nanomaterial synthesis can be done by using green, chemical, and physical methods.

2.1. Synthesis of Cu Nanoparticles

This review is concentrated on various chemical methods involved recently in the synthesis by varying different experimental conditions and the materials can be categorized into copper and copper-based nanoparticles further into: (i) copper and copper oxide (CuO) nanoparticles, (ii) hetero metal doped copper nanomaterials, (iii) graphene oxide-copper nanomaterials, and (iv) copper-based metal-organic frameworks.

2.2. Synthesis of Copper and Copper Oxide Nanoparticles

There are two widely used approaches for synthesizing these nanomaterials that include atomic-level precursors are utilized to synthesize nano-sized material and bulk solids broken into smaller components. The second approach is widely used for its advantage in controlling the shape of the nanoparticles [12,13,14,15]. The synthesis of copper and the copper oxide nano particles are designed based on the final derivative which centers or depends on four chemical reactions namely oxidation, reduction, hydrolysis, and condensation. A diagram outlining the formation of CuO nanoflakes through nucleation growth, orientation attachment, and the Ostwald ripening process is presented in Figure 1.

The following table gives about the various chemical synthesis procedures adopted for the synthesis of copper nano particles. Depending on the desired final nanomaterial, the synthesis can be proceeded with different precursors, the reaction environments, and adopted synthetic methods (such as wet chemical, reverse micelle, Microwave-assisted, etc.) can be changed and is tabulated in Table 1.

2.3. Synthesis of Heterometal Doped Copper-Based Nanocomposites

Various loadings of metal doping are done in nanomaterial synthesis to tailor the materials’ cost, availability, dispersibility, control the morphology, adjust the materials’ conductivity, mechanical and magnetic properties, etc.

Khlifiac et al. [27] describes heterometals (Fe, Mn, Co, Zn and Ni) are doped to CuO which is designated as CuO:Fe, CuO:Mn, CuO:Co, CuO:Zn and CuO:Ni respectively are synthesized using copper sulfate as the primary precursor through coprecipitation method.

CuO nanoparticles can be synthesized by chemical reduction method with the aqueous solution of 1M copper sulfate solution and sodium hydroxide, while maintaining the pH and temperature with constant stirring and other experimental conditions. A similar procedure is followed for doping of all the hetero metals into CuO. The precipitate hence formed is annealed at 500°C for 4 hours to obtain a pure nanomaterial to enhance the crystalline quality of the synthesized powder. In another modified chemical reduction [28] method for the synthesis of Cu₂O nanoparticles, the dopant elements such as La, Mg and Mn are doped while using the 20mM ethanolic solutions of lanthanum nitrate, magnesium nitrate and manganese nitrate respectively [29,30,31]. Ce doped CuO is synthesized by using microwave irradiation method to find microstructural changes from spherical to rod-like structure with optical band gap variation from 3.63 to 3.13 eV [32]. A study disclosed the synthesis of Zr doping on CuO via the Pechini method and enhanced antibacterial properties were investigated [33].

Similarly, many synthesis methods are adopted to obtain doped Cu₂O nanostructures such as electrodeposition [34], chemical displacement [35], hydrothermal synthesis [36], biosynthesis [37,38], chemical vapour deposition [39],[40], pulsed laser deposition [41], solvothermal synthesis [42,43] and spray pyrolysis [39,44]. Table 2 reports the various experimental conditions for the synthesis of heterometal doped copper nanomaterials.

2.4. Synthesis of graphene-oxide (GO) based Copper nanocomposites

For the synthesis of efficient heterogeneous catalysts, graphene has been widely used as a stable and suitable substrate for Nanocatalysts [54,55]. During transformations, its high conductivity can accelerate up the transfer of electrons. [56]. As a result, metal Nano catalysts based on graphene may perhaps promote electrons to facile the reduction efficiency. In fact, when graphene is combined to less reactive Nano catalysts can also produce highly active hybrid nanocomposite catalysts [57]. In an article, CuCl₂ and GO solutions were taken as precursors and synthesized by using hydrothermal process by heating up to 150 °C for 12 h to obtain CuO-GO nanocomposite. The catalyst is affective for the successful reduction of nitro aromatics [58].

In another synthesis of rGO-Ag nanoparticles, AgNO₃ is used and rGO solution is added to AgNO₃ and reduced while using Na [BH₄]. For the rGO-Cu nanoparticle synthesis, AgNO₃ was replaced with Cu (CH₃COO)₂. [59]

In another modified synthesis [60] for non-enzymatic biosensor to detect glucose, the GO-CuO-FTO nanocomposite is prepared. The initial synthesis is comprised of nano belt formation of GO-CuO with a hydrothermal reduction of GO and CuO. This is followed by preparing GO-CuO-FTO prepared using a fluorine-doped tin oxide (FTO) substrate. The following Table 3 reports the recent procedures and the experimental conditions for the synthesis of GO-based copper nanomaterials.

2.5. Synthesis of Copper-Based Organic and Metal-Organic Frameworks

In a study, copper metal–organic framework (Cu-MOF) has been synthesized via hydrothermal method from Cu(NO₃)₂·3H₂O and diphenylamine ligand following a solution-based method. The experimental procedure was regular; however, the homogenous mixture was kept for 3 days aside to get the precipitate [71].

In the preparation of CuO Nanoparticles/Ti₃C₂T_x MXene, the solution containing CuO nanoparticles was placed in an ultrasonic bath and sonicated at room temperature for 20 minutes in order to disperse them. Afterwards, a designated amount (10, 20, 30, 40 wt %) of Ti3C2Tx MXene powder was added to the solution, followed by stirring at 500 rpm for 10 minutes. The mixture was then filtered, washed with ethanol, and dried at 70 °C for 12 hours. The sample with 30 wt. % of Ti3C2Tx-MXene was chosen for further characterization [72].

Development of an electrochemically activated copper nitroprusside (CuNPr)-based sensor for the ultra-trace detection of acetaldehyde (AcH). 0.1 M sodium nitroprusside was added dropwise to a copper chloride solution of equal concentration and stirred for 30 minutes at room temperature. Thus, the formed greenish-blue colloid precipitate was washed and then dried at 50°C. The oxidation of AcH to acetate ions by CuNPr was studied using in-situ Spectro electrochemical analyses. The sensor CuNPr/GCE exhibited a limit of detection towards AcH as low as 41 × 10^{− 8} M. The sensing is successfully employed on red wine sample [73].

A study demonstrated the potential of (4-hydroxyquinoline)4HQ-rGO/Cuas a room-temperature acetic acid gas sensor in practical application. This research studied a graphene-based composite, 4HQ-rGO/Cu²⁺, prepared through supramolecular assembly of graphene nanosheets, 4-hydroxyquinoline (4HQ), and copper (II) ions. When acetic acid was attached, the supramolecular assembly showed an enhanced sensing performance at room temperature due to the accelerated charge transfer between the graphene nanosheets and 4HQ molecules. The copper (II) ions also acted as the main active site for gas adsorption and the as-fabricated sensor exhibited a high response time within 5s at room temperature [74].

A research study has focused on the magnetic adsorption material polyaniline (PANI) with an amino functional group combined with CuFe₂O₄ (CuFe₂O₄/PANI nanocomposite). Coprecipitation technique is adopted for preparation of CuFe₂O₄ nanoparticles. CuFe₂O₄/PANI nanocomposite was prepared by chemical in situ polymerization. The material showed that it has an extremely high maximum adsorption capacity of 322.6 mg/g for the removal of uranyl ions from wastewater at a pH of 4. The adsorption process followed the quasi-second-order kinetic equation. Also stated that the material has stable adsorption performance for uranyl ions after five cycles of recovery in acid medium [75].

The study presents a novel approach for the synthesis of Pd-Cu alloy nanoparticles encapsulated in carbon nanopillar arrays (Pd–Cu@HPCN) which are promising oxygen evolution electrocatalysts. Figure 2 shows by using Cu-based MOF materials as a framework, the author disclosed a versatile technique to produce a Pd-Cu alloy enclosed within porous carbon nanopillar arrays.

Figure 2 shows the synthesis involves the preparation of a core-shell structured MOF@imidazolium-based ionic polymers (ImIPs) template and the subsequent decomposition of the inner Cu–MOFs when an anion exchange occurs between sodium tetrachloropalladate in solution and bromides in the external ImIP shell. The resulting Pd–Cu@HO-ImIP array is then topotactically transformed to generate Pd–Cu@HNPC [76].

3. Characterization Methods for Copper-Based Nanomaterials

Various techniques for the characterization of copper oxide and its composites were found by using elemental composition analysis (such as Scanning electron microscopy, Energy dispersive X-ray spectroscopy EDS), structural characterization (such as X-ray diffraction) Morphological analysis (such as TEM), Surface area and pore size distribution (such as Brunauer–Emmett–Teller (BET) measurements) and optical properties (such as UV-Vis spectrophotometer, UV-Vis-Near IR) [31].

Various techniques have been used for the characterization of nanomaterials based on the properties such as size and shape, etc. which are needed to be measured and that are characteristic of the nanomaterial which can be used for further reproducing the experiments. Figure 3 shows the copper nanoparticle characterization techniques [77].

4. Copper-Based Nanomaterials for Environmental Pollution Management

4.1. 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. Due to this high usage of dyes which are the main sources of organic pollution in various industries lead to a global concern [78,79,80]. 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 [78,81]. Hence removal of these effluents is important. Therefore, a sustainable technique for the removal of such effluents is essential to get rid of these hazardous pollutants and therefore efficient catalytic reduction and photocatalytic degradation are essential [82,83,84]. The two oxidation states of copper Cu²⁺ and Cu⁺ acts as electron trapper leading to 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 [85]. 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 [86], and also showed reduction of 4-nitrophenol [87].

Hamed et al. concluded that the copper nanoparticles decorated alginate/cobalt-doped cerium oxide composite beads are a promising photocatalyst for the reduction and photodegradation of organic dyes. his composite material is effective at degrading a wide range of organic dyes, with no additional energy source required [88]. Table 4 shows the comparison of the photocatalytic efficacy of the copper-based nanomaterials concerning various dye pollutants.

4.2. Copper in Reduction of other Heavy Metals 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. which do not break down further in the environment and accumulate into the vital organs of the animals and cause various chronic diseases and death in extreme cases [97,98,99,100,101].

A highly porous material is synthesized Cu-DPA MOF [102] for the removal of heavy metals from wastewater. The adsorption parameters such as pH value, contact time, initial metals concentration, 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 is determined through batch adsorption experiments. An optimized procedure is performed in order to carry this procedure on wastewater containing Cd, Cr, Pb [102]. An overview of some of the copper nanomaterials associated with the removal of heavy metal ions are tabulated in Table 5. The adsorption capacity and removal efficiency were calculated using equations [i, ii] respectively, where q_e is heavy metal ions concentration adsorbed on adsorbent at equilibrium (mg of metal ion/g of adsorbent), C_o and C_e are the initial and equilibrium concentration or final concentration of metal ions in the solution (mg/L), V is the initial volume of metal ions solution used (in L) and m is the mass of adsorbent (in g).

$${\mathrm{q}}_{\mathrm{e}}=\frac{\left({\mathrm{C}}_{\mathrm{e}}-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{m}}$$

(i)

The removal efficiency was calculated using Eq. (ii), where C₀ and Ce are the initial concentration of heavy metals (mg/L) and the equilibrium concentration of heavy metals (mg/L), respectively.

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=\frac{({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{C}\mathrm{o}}\mathrm{X}100$$

(ii)

4.3. Copper-Based Nanomaterials in Wastewater Treatment

As a result of rapid industrialization, the volume of pollutants had been increased apparently. Industrial by-products often include dangerous and cancer-causing synthetic organic dyes, pesticides, pharmaceuticals, and textile waste. If these materials are not disposed 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. This can lead to serious environmental issues due to the compounds’ ability to remain stable in the environment [25].

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 been evolved [111]. Due to the pore size, the high surface area of the nanomaterials that has unique properties such as photosensitivity, antimicrobial activity, catalytic activity, magnetic, electrochemical, and optical properties provide a wide range of applications in the field of remediation of pollutants, detection, and water quality monitoring [112,113]. Many persistent pollutants after long-term exposure cause chronic diseases in animals and humans. For instance, 4-nitrophenol is widely used in the manufacture of drugs, dyes, insecticides and fungicides, and leather industries. Organic material has acute effects in humans that cause headaches, nausea, cyanosis, and irritation to the eyes [114,115]. Detection of such organic chemicals is highly needed in drinking and other sources of water. In a study, the copper oxide-reduced graphene oxide nano composite is synthesized for the enhanced catalytic activity towards reduction of 4-nitrophenol [116].

A research study discusses the synthesis of CuS QDs@ ZnO hybrid nanocomposites as an environmentally friendly preparation to improve the performance of the ZnO nanorods photocatalyst for the degradation of dyes, pharmaceuticals, and pesticides in water under simulated sunlight. The XRD, SEM, and TEM analysis results exhibited the CuS QDs@ZnO hybrid nanocomposite had a high crystallinity and smaller sphere size of 2 nm. It was further found that the crystallinity and light absorption as well as degradation activity increased with increasing the ratio of CuS up to 3%, but then decreased with further increase. The 3% CuS QDs@ZnO hybrid nanocomposite had the capability to reduce the electron–hole recombination rate, which enhanced its degradation rate of organic pollutants. [25]

4.4. 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 [60]. In the presence of phosphate buffer solution at pH 7.0, glucose sensing is performed. It is drafted that the prepared sensor exhibited excellent electrical conductivity, and low detection limit in human serum in comparison, which is perhaps due to the large superficial area that leads to good catalytic activity. The glucose sensing mechanism is given in Figure 4.

The study of the biological process called DNA methylation is important, without changing the sequence, the activity of the DNA segment can be changed with methylation. DNA methylation often inhibits the transcription of genes when it occurs at a gene promoter. DNA methylation is crucial for proper growth in mammals and also associated with aging, genomic imprinting, carcinogenesis, X-chrome inactivation, etc… In a study, modified reduced graphene oxide (rGO) which is decorated on CuNPs is 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 [117]. Table 6 provides an outline of the recent research showing the copper nanomaterial’s response towards bioanalytes.

Some small molecules such as H₂O₂ is a key players in the design of many biosensors. It is a substrate in enzymatic reactions and can be used as an electron mediator or oxidizer in electrochemical reactions. It is also used to detect the activity of certain enzymes, such as glucose oxidase, in biosensors. Additionally, H₂O₂ can be used to detect the activity of certain proteins, such as cytochrome c, in biosensors. [118,119].

4.5. Copper-Based Nanomaterials in Pesticides Remediation in Soil

Despite being officially prohibited in many countries, several pesticides and insecticides are still in use today, for instance, endosulfan and carbofuran [126]. One of the most frequently used sulfur-containing organic molecules is dithiocarbamates with major applications in the production of sugar, rubber manufacture, antioxidants and antislime in paper making [127,128]. Between 25,000 and 35,000 metric tons are estimated to be consumed annually in this manner [129,130]. These dithiocarbamates are categorized into different classes such as zineb, ferbam, maneb, etc. based on the carbon skeleton and properties [131]. In a report article, an advancement demonstrated for sensing dithiocarbamates (DTCs) Ziram, Zineb, and Maneb pesticides by using cetyltrimethyl ammonium bromide (CTAB) capped copper nanoparticles as the colorimetric probe. This economical probe is used for the detection of these pesticides in various juice samples [132]. In recent advances in agrochemical research, nano fertilizers provide nutrients to plants and even replace many fertilizers, to improve crop yield and quality. Bollworm is a serious pest in the cultivation of cotton. In a study, an engineered CuNPs has the potential of insecticidal activity in as low dose (10mg/L) to regulate the exogenous bacillus thuringiensis microbial protein coded through BT toxin in plant tissue to improve resistance against bollworm [133].

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

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 [134].

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 CECs molecules into harmless by-products. Similarly, another approach to detoxifying CECs is Nano remediation. Nano remediation involves the use of nanomaterials, such as nanoparticles, to absorb and trap the CECs molecules. Nanoparticles can be engineered to specifically target CECs 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 CECs molecules more effectively by using physio-chemical treatments that include adsorption, oxidation, and filtration. Adsorption involves using particles, such as activated carbon, to absorb the CECs molecules from an environment. Oxidation involves using chemical oxidants, such as ozone, to break down the CECs molecules into harmless by-products. Filtration involves the use of membrane filters to remove CECs molecules from an environment. Overall, the most effective and efficient way to detoxify CECs depends on the specific environment and the type of CECs molecules present. In some cases, a combination of different strategies may be needed to achieve the desired results. All strategies used to detoxify CECs must be environmentally friendly and use sustainable resources [135].

For example, reverse micelle synthesized Cu-TiO₂ nanomaterials towards levofloxacin under visible light emitting diode (LED) light. The reaction rate constant of the nanomaterials was 0.0347 min⁻¹, and the highest degradation efficiency achieved was 93.3%. The results indicate that the nanomaterials were able to adsorb, oxidize, and degrade levofloxacin under visible LED light [48].

Table 7 summarizes the performance of copper-based nanomaterials used in pharmaceutical drug degradation. The recorded concentration of the drug, catalyst loading, temperature/pH, and degradation source, degradation efficiency were all given. The results suggest that copper nanomaterials are promising for the degradation of pharmaceutical drugs.

4.7. Copper-Based Nanomaterials as VOCs Sensor

VOCs (Volatile Organic Compounds) 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 [142]. 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 [143]. 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, kidney failure, hepatic encephalopathy, and liver dysfunction [144]. 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. Table 8 shows various techniques for deposition and applications of copper sensors to sense volatile organic compounds (VOCs).

4.8. Copper-Based Nanomaterials in Carbon Dioxide Reduction

CO₂ electroreduction (ER) 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 [153].

Use of CuO-ZnO nanomaterials as catalysts to convert carbon dioxide into methanol. This study determined that bimetallic systems combined with porous supports, such as zeolite and activated carbon, had a greater efficiency when compared to unsupported materials. The hydrogenation at different temperatures is carried in a stainless-steel-packed bed reactor for the conversion to methanol which indeed is to reduce the environmental emissions of carbon dioxide emissions [154].

In certain CO₂ reduction experiment, which is conducted electrochemically in two compartments of H-cell in ethylene production, while copper oxide nanoparticles as the catalyst. The ethylene production is dependent on the morphology of the catalyst. The CuO nanoparticles are deposited on conductive carbon materials which will be activated, and the copper species converted to Cu+ which eventually results in the formation of 70% ethylene and 30% of Hydrogen Faradaic efficiency (FE) without any other by-products in an aqueous solution [155].

Listed data reveals the surface morphology of various copper catalyst surfaces on the faradaic efficiency of carbon products that had been reported in Table 9.

Figure 5 shows CO₂ reduction to ethanol production over Cu/Cu_xO PCC electrocatalyst at low over potential. The amount of Cu/Cu_xO nanoparticles embedded in the carbon-nitrogen network turned into altered through varying the leaching time with nitric acid. Leaching the cuboids for 1 h (Cu/Cu_xO-PCC-1h) led to 1.93 at% copper and leaching for 6 h (Cu/Cu_xO-PCC-6h) ended in 0.86 at%, while the unleached cuboids showed 16.93 at% Cu content material determined from evaluating copper to carbon peaks in XPS spectra. The leaching effect was studied towards CO₂ reactivity and turned into proven to significantly increase the materials’ surface area, increase the porosity, adjust the nitrogen nature, and dissipate the Cu nanoparticles. The impact of these parameters became meditated inside the materials’ electrochemical performances. It is display that improving the materials’ porosity and surface region cannot have a positive effect without owning sufficient number of catalytic active sites. Similarly, pyridinic nitrogen content regarded to be correlated to improved electrocatalytic overall performance.

5. Conclusions

In conclusion, copper-based nanomaterials have shown great promise for environmental remediation such as waste water 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 shall be extended in different areas of synergetic effects in environmental management in the areas of reduction, degradation, reuse, recycling, etc. With further research, copper-based nanomaterials could be used to effectively clean up many types of contaminated environments, ultimately leading to a healthier and safer world for all of us.