Green Synthesized Carbon-Based Nanomaterials: Applications and Future Developments

ANU ROSE CHACKO, NEENA JOHN PLATHANAM, BINILA К KORAH, THOMAS ABRAHAM, and BEENA MATHEW*

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

*Corresponding author. E-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

ABSTRACT

Carbon-based nanomaterials are the most valuable materials used in the modem field due to its high potential of application in almost all areas of living. Increase in crisis for energy and environmental degradation are the foremost challenges of using nanotechnology for sustainable development. Here lies the importance of green nanotechnology, which focuses on the use of green natural precursors for the development of eco-friendly processes and products. In this scenario, researchers have started the use of renewable, inexpensive, and abundant carbon-based nanomaterials for sustainable development. Even though there are enormous applications of carbon-based nanomaterials in various fields, this chapter proceeds with the applications of green synthesized nanomaterials for future perspectives. The major applications of carbon-based nanomaterials outlined involve prevention of environmental degradation, improvement of public health, energy efficiency, optimization, and industrial development. Green synthesized carbon nanomaterials such as carbon dots, carbon nanotubes (CNTs), graphene, fiillerenes, and nanodiamonds are given special attention due to their excellent and efficient properties. This chapter provides the reader the current progress, highlighting the application in environment and energy-related fields of the carbon-based nanomaterials synthesized using green protocol.

INTRODUCTION

Carbon-based nanomaterials with its accountable properties and enormous applications have become an unavoidable part of development. They play a chief role in the prevention of environmental degradation, improvement of public health, energy efficiency optimization, wastewater reuse, and pollutant transformation. But the fossil fuel-based precursors used for the synthesis of carbon nanomaterials are a major challenge in light of the increase in global demand for energy. In order to address these issues, there is a high need for innovative and efficient sustainable solutions. Production of nanomaterials that could solve environmental problems without causing any harm to the environment or living organisms are considered as the essentialities for safer green nanotechnology. Nontoxic precursors which are renewable, eco-friendly, and inexpensive are excellent alternatives for traditional chemical sources.¹-²

In this chapter, efforts are done to emphasize the applications of carbon-based nanomaterials that are synthesized from natural green precursors. And also highlight the fact that these green synthesized carbon nanomaterials could give better outcome compared to fossil fuel-derived carbon nanomaterials. This chapter proceeds with a main focus on the specialties and applications of green synthesized carbon nanomaterials such as carbon dots, CNTs, graphene, graphene oxide (GO), fullerene, and nanodiamond. Figure 6.1 illustrates all the potential applications of the green synthesized carbon-based nanomaterials. We expect that this will provide a brief compilation of all the applications and trends in the use of green synthesized carbon nanomaterials.

CARBON QUANTUM DOTS (CDs)

Fluorescent CDs are a new class of carbon nanoparticles that have recently emerged and have gained much interest as competitors of traditional semiconductor quantum dots (QDs). The observable superior properties of CDs include water solubility, low toxicity, biocompatibility, small size, fluorescence, and ease of modification. Zero-dimensional, size of <10

шп and quasi-spherical CDs have attracted much attention since 2006, as an environmentally friendly substitute for toxic QDs.³ For many years, semiconductor QDs have been extensively investigated for their durable and tunable fluorescence emission properties, enabling their applications in sensing and bioimaging. However, semiconductor QDs have some limitations such as high toxicity due to the use of heavy metals in their production. Heavy metals are known to be veiy toxic even at relatively low doses, which would preclude any clinical study.⁴-⁵ This was solved since 2004, with the invention of CDs having similar fluorescence properties.⁶

FIGURE 6.1 Schematic illustration of applications of green synthesized carbon nauomaterials.

The strategies for the synthesis of CDs are based on cutting larger carbon materials (top-down) or fusing smaller precursor molecules (bottom-up).³-⁴ Generally, the bottom-up method has a high yield and it is convenient to introduce heteroatom doping in the synthesis process. Various methods of preparing CDs have been developed and most studies attempt to produce high-quality CDs using simple, cost-effective, size-controlled, or large- scale synthetic methods.⁷ The preparation of organic CDs usually adopts the bottom-up approach including hydrothermal,⁸ microwave treatment,⁹ pyrolysis,¹⁰ extraction,¹¹ etc. Among them, hydrothermal treatment is the most commonly used method. Organic carbon sources are eco-friendly natural products compared to other carbon sources and they have many advantages in making CDs. It is inexpensive, easy to get, green, and harmless. In addition, the production of CDs from natural biomass can turn low-value organic waste into valuable and useful materials. Organic precursors play an important role in the entire energy system.

APPLICATIONS

As a group of newly emerged fluorescent nanomaterials, CDs show tremendous potential for a variety of applications including chemical sensing,¹² biosensing,¹³ bioimaging,¹⁴ drug delivery,¹⁵ photocatalysis,¹⁶ and electrocatalysis,¹⁷ due to its excellent photoluminescence (PL) properties and ease of modifying their' optical properties through doping and functionalization. Traditional fluorescent labeling materials have the disadvantages of a complex synthesis process and are easily self-assembled in the aqueous phase, which limits their application. Compared to conventional fluorescent dyes, CDs have higher stability and are more easily dispersed with water. Over the past few years, there has been tremendous progress in the use of renewable, inexpensive, and green resources not only addressing the urgent need for large-scale synthesis of CDs but also promoting the development of sustainable applications.

CHEMICAL SENSING

An interesting application of CDs is in the field of sensing. In principle, any PL changes including PL intensity wavelength, anisotropy, or a lifetime in the presence of different concentrations of specific analytes are likely for application in fluorescence-based sensing. It is done with the interaction of analytes and chelating agents on the surface of CDs designed through the introduction of functional groups (either from the precursors or solvent) during the synthesis process, postfunctionalization or integration with other molecules such as quenchers or fluorophores.¹⁸ CD-based fluorescence sensing can be attributed to various mechanisms including photo-induced electron transfer,¹⁹ fluorescence resonance energy transfer,²⁰ inner filter effect,²¹ electron transfer,²² aggregation-induced emission enhancement effect,²³ aggregation-induced emission quenching effect,²⁴ static quenching effect,²⁵ and dynamic quenching effect.²⁶ These principles make CDs a great candidate for the detection of heavy metals, cations, anions, biomolecules, biomarkers, nitroaromatic explosives, pollutants, vitamins, and drugs.

Fe^3_ are the most common detected ions since the functional groups such as ammo groups, carboxyl groups, and hydroxyl groups are generally present on the surface of CDs and thus can be effectively combined with iron ions. Yang et al. reported a procedure for selective detection of Fe³⁺ from honey.²⁷ Coriander leaves,²⁸ egg white,²⁹ garlic,³⁰ sugar cane molasses,³¹ rose-heart radish,³² grape peel,³³ and other diverse biomass are used to prepare CDs for n on ion detection. The detection of heavy metals such as Hg²⁺ is crucial because of their hazardous effect on the environment and human health. Green CDs have been employed for selective and sensitive Hg²⁺ detection on various occasions. Various precursors such as pomelo peel,³⁴ flour,³⁵ strawberry juice,³⁶ tea,²⁶ urine,³⁷ hair,³⁸ edible mushrooms,³⁹ etc. are used to detect mercury ion from the water bodies. CDs prepared fr om biomass precursors such as bamboo leaves,⁴⁰ Осипши sanctum,⁴¹ pigskin,⁴² peach gum,⁴³ and lemon peel¹⁶ are widely used for the detection of other various cations, such as Cu²'. Pb²⁺, Co²⁺, Au³⁺, and CrY

Compared to cations detection, there are few reports about the use of biomass CDs for anion detection. Various cations can quench the fluorescence of biomass CDs. After adding some specific anions, the fluorescence was recovered because of the binding between cations and anions. This on-off phenomenon can be used to detect many different combinations of cations and anions including (Cu²⁺-S² ), (Fe³"-S,0₃²'),⁴⁴ (Fe³⁺-P0₄³ ), etc. and CDs detect some other anions by quenching mechanisms, for example, CIO',⁴⁵, CN etc. Xu et al. synthesized blue-fluorescence CDs from potatoes for the detection of P0₄³' ion.⁴⁶ In addition to detecting ions, CDs can also be used to detect molecules, including tartazine,⁴⁷ tetracycline,⁴⁸ glutathiones,⁴⁹ etc. Organic CDs have appreciable selectivity and sensitivity which may lead to the fabrication of devices for real-time detection.

BIOIMAGING

Bioimaging is an intriguing application for biomass CDs. The extremely small size of CDs means they can be taken up easily by cells which can be imaged for intracellular fluorescence. These biomass CDs have been used to culture living cells such as HaCaT cells,⁵⁰ E. coli,⁵¹

HepG2 cells⁵² to test their potential for dual-model fluorescence/MR imaging. In addition to the effect of biocompatibility of biomass CDs, the preparation of CDs with various emission wavelengths also plays a vital role in bioimaging. Park et al. used mango fruit as a carbon source to synthesize CDs. The obtained CDs exhibited blue, green, and yellow fluorescence which were used as a probe for bioimaging.⁵³ Moreover, the size of biomass CDs also has a certain impact on cell imaging. The small size biomass CDs are more easily captured by cells and contribute to intracellular fluorescence imaging. Shi et al. used various plant petals as carbon precursors to prepare CDs for cellular imaging of human uterine cervical squamous cell carcinoma (A193).⁵⁴ Because of the specificity of cells, pH value is also a significant factor affecting biomass CDs bioimaging. Shuang et al. produced fluorescence CDs derived from leeks to study Cu²⁺ and pH sensing in cells. The CDs with pH-dependent behavior exhibited turn-on fluorescence as pH ranging from 3 to 11.⁵⁵ Although the development of CDs is still in its infancy, CDs have great potential in bioimaging and promote the development of biomedicine.

CATALYSIS

Catalysis is the next major area of application of organic CDs. Researchers always doped natural CDs with other materials and the as-composited CDs can be used in electrocatalysis and photocatalysis. Because of their photoluminescent properties and photoelectron transfer properties, CDs can be considered an active ingredient in the manufacture of high- performance photocatalysts. Heteroatom doping functionalization of CDs can adjust the bandgap and increase the quantum yield and have a certain catalytic performance of the CDs. Tyagi et al. prepared CDs from lemon peel waste using a facile and cost-effective process. The photocatalytic activity of the TiO,-CDs composites was confirmed by immobilizing the synthesized CDs on the electrospun TiO, nanofibers and using methylene blue (MB) dye as a model pollutant. The photocatalytic activity of the TiO,-CDs composite is approximately 2.5 times greater than that of the TiO, nanofibers.¹⁶

DRUG DELIVERY

Biomass CDs are highly fluorescent, excellent biocompatible, rapid cellular acquisition, and high stability, so they serve as a multifunctional vehicle for drug loading and release. D’souza et al. used dried shrimp as the source material of CDs and were reasonably made into a traceable drug delivery system for targeted delivery of crude in MCF-7 cells.⁵⁶ Mehta et al. synthesized CDs by using pasteurized milk as a carbon source. Then, they prepared lisinopril-loaded CDs by self-assembly of lisinopril on the surfaces of CDs.⁵⁷ The abundance of chemical groups such as amino or hydroxyl groups on CDs can promote their future functionalization. These highly biocompatible CDs can serve as a novel fluorescent tool for studying drug delivery.

SOLAR CELLS

The earth soaks up to 86 PW of solar energy every year and harnessing this energy using photovoltaic devices is one of the keys to making the world carbon neutral. Unfortunately, current photovoltaic devices are limited by their high manufacturing costs and low efficiency. One possible solution is to use organic CDs in a new generation of solar cells since they show excitation-dependent or excitation-independent fluorescence emission and size-dependent fluorescence emission. Zhang et al. reported a fluorescence quenching mechanism that significantly improved the conversion efficiency of CDs sensitized aqueous solar cells. CDs synthesized from grass were chosen as the test case to validate the principle proposed to be responsible for enhancing the fluorescence quenching.⁵⁸

OTHER FIELDS

hi addition to some of the above-mentioned significant and wide range of applications, there are some other applications such as light display materials, anti-counterfeiting materials, and confidential materials. Zhang et al. presented an investigation of CDs synthesized from tofu wastewater. This study is relevant to the application of fluorescent CDs as light display materials.⁵⁹ Liu et al. used hair as a carbon source to produce highly fluorescent CDs by a one-step pyrolysis treatment. These CDs are useful in fluorescent patterns, flat panel displays, and anti-counterfeiting labeling. The electron-donor capabilities of photoexcited CDs have clear potential in reduction reactions.⁶⁰

Carbon dots have proven benefits over conventional QDs and organic fluorophores for various applications resulting from then easy synthesis, low cytotoxicity, and superior optical properties. However, biomass CDs with high quantum yields (QYs) are still less reported. In addition, improving sensitivity and selectivity is a challenge. Based on the unique characterization of CDs, various types of probes were fabricated to detect various metal ions, anions, small molecules, and macromolecules by observing the process of quenching or recovery of the fluorescence of CDs. Another exciting area for further development is specific targeting of cellular organelles with green CDs. The CDs-based bioimaging probes could be tuned for high intracellular photostability for long term imaging. Another important application is in the field of electronics. Organic CDs are the future of electronic devices with their high performances including biocompatibility, nonphotoblinking, and excellent fluorescence properties. They have potential for use in various applications including biosensors, organic light-emitting devices, energy storage devices, and organic photovoltaics. These qualities have inspired great vision in the preparation and application of biomass CDs, helping to achieve green chemistry. The remarkable progress in developing different organic CDs for a variety of applications is actually what separates them from other carbon structures and makes them the next generation material.