Keywords

1.1 Carbon Dots (CDs)

Recently, carbon- or graphite-based quantum dots have gained growing attention in environmental applications, owing to its unique optoelectronic properties, broadband optical absorption, bright fluorescence emissions, favorable photoinduced electron transfer properties, and cost-effectiveness in synthesis routine (Yan et al. 2016). Accidentally discovered by Xu et al. in 2004 during purification of single-wall carbon nanotubes (SWCNTs) (Xu et al. 2004), carbon dots (CDs) have recently emerged as a new class of semiconductor due to its distinct properties, e.g., excellent photostability, excitation wavelength dependent fluorescence, low toxicity, effective infrared-responded upconverted photoluminescence (UCPL), and tunable fluorescence emission (Essner et al. 2016; Sachdeva and Gopinath 2015; Jelinek 2017). They are known as quasi-spherical shape nanoparticles with sizes below 10 nm that encompass a carbonegic core with surface functional group (Wang et al. 2017). CDs consist of graphitic carbon (sp2 carbon) and graphene oxide sheets where the diamond-like sp3-hybridized carbon is present, stabilizing the CDs three-dimensional network (Demchenko and Dekaliuk 2013). The carboxyl groups on the surface of CDs provide great water solubility and enable the advance functionalization with chemically reactive group and surface passivation with organic, inorganic, polymeric, or biological materials to CDs that could enhance fluorescence properties of CDs (Lim et al. 2015; Baker and Baker 2010).

CDs possess excellent fluorescence characteristic and high photostability against photobleaching and blinking which potentially become an alternative to heavy metal-based semiconductor quantum dots (SQDs) currently in use (Wang et al. 2011). Although SQDs like CdSe, CdS, and PbSe were widely used due to their strong photostability, fluorescence, and tunable emission, their high toxicity has raise safety concern of environment and human’s health, restricting both biological and environmental application. The differences between CDs and SQDs are that luminescence of CDs emits from the surface, whereas in the latter case, luminescence originates from the core of the nanocrystals (Štěpánková et al. 2015). So by attaching the carboxyl, hydroxyl, and amino groups on the surface of CDs, the optical property and biocompatibility as well as the selectivity and sensitivity could be enhanced. CDs are able to emit PL under near-infrared (NIR) light excitation that given a potential for light energy conversion, photovoltaic devices, and related applications (Wang et al. 2009). The PL intensity of CDs tends to reduce when the pH turns alkaline, making it suitable for cellular imaging (Pandey et al. 2013). CDs are also used as bio-sensing for visual monitoring of glucose, potassium, nucleic acid, copper, and pH (Namdari et al. 2017).

The photoluminescence (PL) of CDs is dependent on quantum effect of nanoparticles with various sizes, emissive traps on the surface of CDs, excitation wavelength (λex), or other unknown factors (Wang and Hu 2014; Baker and Baker 2010). The PL of CDs can be efficiently quenched by electron acceptor or donor molecules in solution, indicating that photoexcited CDs exhibit excellent electronic properties as electron donor and acceptor (Zhang and Yu 2015). Besides that, it also can act as an electron mediator, photosensitizer, and spectral converter due to its UCPL property (Wang et al. 2009a). Thus, it is not surprising that the applications of CDs have been extended to photocatalysis of water splitting and removal of organic pollutants in recent years. Nevertheless, the mass fabrication and selection of an appropriate precursor that is low cost, sustainable, high carbon yield, and easily available for long period of time (Zhu et al. 2013) are the main challenges facing by CDs.

1.2 Carbon Dots Synthesized from Green Precursors

Recently, CDs derived from green precursors (green CDs) have gained attention from numerous researchers because of its excellent properties and also environmentally friendly synthesis routine (Yan et al. 2016). The “green precursors” are defined as derivatives of renewable natural products or processes or naturally occurring (Sharma et al. 2017). Green CDs show much more promise in greener and sustainable future development as most of the natural resources can be obtained economically and also synthesized using versatile method (Shen and Liu 2016). In past researches, various chemical precursors such as glucose (Ma et al. 2012), citric acid (Ju et al. 2014; Schneider et al. 2017), ethylene glycol (Wang et al. 2017b), and EDTA (Liu et al. 2017) were widely reported. Since CDs are mainly applied for biomedicine application such as drug delivery, biosensing, bioimaging, and gene transfer, the switch from chemical precursors to “green precursors” is more appropriate. The green precursors can be obtained from natural resources such as sugarcane juice (D’souza et al. 2016; Sim et al. 2018), pomelo peels (Lu et al. 2012), willow bark (Qin et al. 2012), and bagasse (Du et al. 2014) (Fig. 1.1). Hydrothermal carbonization method is widely used to prepare CDs from green materials due to its low cost, nontoxic nature, and environmentally friendly process. The quantum yield (QY) of CDs is based on the type of green precursors and preparation method used, as shown in Table 1.1.

Fig. 1.1
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Green precursors used to produce CDs. (From Sharma et al. 2017)

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Table 1.1 Synthetic method and quantum yield of different precursors
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Nitrogen doping in CDs has been reported recently to increase the QY and to improve the intrinsic low emission efficiency. Liu et al. (2012) successfully increased the QY of grass-derived CDs from 2.5% to 6.2% after doping with nitrogen. One-pot hydrothermal carbonization method was used by Liao et al. (2016) to produce water-soluble nitrogen-doped CDs (N-CDs) from the mixture of natural peach gum polysaccharide (PGP) and ethylenediamine. The QY of N-CDs was enhanced from 5.31% to 28.46% after surface passivation of CDs with nitrogen (Liao et al. 2016). Ethylenediamine was also used as a nitrogen source to produce N-CDs from onion wastes (Bandi et al. 2016) with QY of 28%. Liu et al. (2017a) used ammonia as nitrogen source to dope CDs derived from rose-heart radish and achieved a QY of 13.6%. Ding et al. (2017) used pulp-free lemon juice as intrinsic nitrogen source to prepare CDs with high QY (28%) and intense red luminescence. High-protein sources such as milk (Wang and Zhou 2014), chicken egg (Wang et al. 2012), oatmeal (Yu et al. 2015), hair (Guo et al. 2016), etc. were used as intrinsic nitrogen sources for the self-passivation of CDs. Among these intrinsic sources, oatmeal successfully produced N-CDs with the highest QY (37.4%) due to the highest N/C ratio prepared at 200 °C (Yu et al. 2015). Besides N-doping of CDs, other researchers also produced the nitrogen- and sulfur-co-doped CDs (N-S-CDs). Sun et al. (2013) developed a large-scale synthesis of N-S-CDs by using sulfuric acid carbonization and etching of hair fiber. They claimed that higher reaction temperature at 140 °C was favorable to form N-S-CDs with smaller size, higher S content, and longer wavelength of photoluminescence emissions. A greener method was reported by Zhao et al. (2015), where N-S-CDs were prepared using garlic as intrinsic source for N and S doping without using harmful chemicals like sulfuric acid as sulfur source. The obtained CDs possess good water dispersibility and quantum yield of 17.5%.

1.3 Synthesis

Numerous synthesis approaches of CDs reported can be categorized into two major routes: top-down and bottom-up approaches. CDs can be produced and modified via physical, chemical, or electrochemical techniques during preparation or posttreatment period (Namdari et al. 2017). The unique characteristics of each CDs synthesis method were summarized in Table 1.2. During CDs synthesis, three main issues such as carbonaceous aggregation, size control and uniformity, and surface properties are needed to be addressed to create CDs with optimum properties for various applications (Wang and Hu 2014).

Table 1.2 The unique characteristics of each CDs synthesis method
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1.4 Top-Down Approaches

The top-down route generally involves the breaking down process of relatively large carbon structures such as activated carbon, carbon nanotubes, carbon soot, graphite, graphite oxide, and nanodiamonds via arc discharge, chemical oxidation, and electrochemical and laser ablation (Xu et al. 2014; Liu et al. 2016). The top-down approaches are typically conducted as their simple operation, and abundant raw materials allowed mass production of CDs. However, these approaches require specific treatments and would produce low yield from graphite source as most graphites have relatively large graphitic structures which are hard to be broken down to less than 10 nm in diameter. Dong et al. (2010) found that this drawback can be slightly reduced by altering the carbon source with amorphous carbon form that contained abundant tiny crystalline graphite fragments via a suitable synthesis method. Hence, the discussion of top-down approaches is done to allow better visualization on top-down approaches.

1.4.1 Arc Discharge

Arc discharge approach is a popular method to synthesize carbon nanomaterials due to its high production of low-defect products with environmentally benign properties. The morphologies and productivity of CDs can be controlled with arc satiability and precursor temperature history as they determine the interaction at the anode boundary (Liang et al. 2017). Arora and Sharma (2014) elaborated that the arc discharge process involved the electrical breakdown of a gas to produce plasma by using electric current in alternating or direct current. The generated plasma (4000–6000 K temperature) with high heat flux or thermal energy would then sublime the bulk carbon precursor to create carbon vapors. Next, the carbon vapors would undergo phase change and transform into liquid at cooler temperature. Figure 1.2 shows the schematic drawing of an arc discharge setup. Xu et al. (2014) discovered the production of stable black and crude suspension with arc-discharge soot oxidation with nitric acid precursor and sediment extraction with sodium hydroxide aqueous solution. The black suspension separated an average of 18 nm diameter fluorescent CDs after gel electrophoresis purification method. Moreover, Bottini et al. (2006) showed that the blue to yellowish-green fluorescent range CDs were produced via arc discharge from carbon nanotubes. The discovery of argon gas breakdown in a fluid form for CDs production was pioneered by Sun et al. (2016) and allowed the elimination of agglomerated CDs by frequency adjustment. Despite that, CDs yield via arc discharge approach is still low (Zuo et al. 2015).

Fig. 1.2
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Schematic drawing of an arc discharge setup. (From Arora and Sharma 2014)

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1.4.2 Electrochemical Carbonization

Electrochemical carbonization method is another widely employed nonselective method in top-bottom approach (Zhao et al. 2008; Ming et al. 2012). Bulk carbons such as carbon nanotubes, carbon fiber electrodes, and graphite are used in the production of carbon quantum dots. Deng et al. (2014) employed a conventional three-electrode system composed of two Pt sheets with a dimension of 4 × 4 cm2 as working and counter electrode and reference electrode. The reference electrode is a calomel electrode that is mounted on freely adjustable Luggin capillary. The applied voltage in the setup can affect size, carbon content, and UV-vis absorption and brightness of CDs similar with Bao et al.’s (2011) study. Another study done by Li et al. (2010) showed that the electrochemical carbonization synthesis of CDs was assisted by alkali. An alkaline environment played a major factor as it aids in judicious cutting of carbon honeycomb layer into ultrasmall carbon particles due to the presence of hydroxyl (OH−) group from alkali suspension. Zhou et al. (2007) reported that the structure evolution of bulk carbon changed upon the number of cycles of electrochemical treatment. Multiwalled carbon nanotubes at 100th electrochemical treatment cycles had entangled with curled and swelling features, while 1000th electrochemical treatment cycles had caused serious deformation and tube wall openings. Therefore, the number of electrochemical treatment cycles influenced the active area and surface reaction of CDs. In comparison with other methods, this method reduces synthesis reaction time and does not need costly reagents, further surface modification, high temperature, and strong acid (Hou et al. 2015). However, the quantum yield of CDs is low with electrochemical carbonization (Zuo et al. 2015).

1.4.3 Laser Ablation

According to Shahidi et al. (2018), the laser ablation method is a convenient, cheap, and trouble-free method. This method utilizes pulsed laser with high pulse energy on target surface immersed in a liquid medium for vaporization and flume formation which contains high kinetic energy cluster, as well as atomic and ionic particles (Gondal et al. 2013). The flume would react with the liquid medium to produce CDs. However, the laser ablation in liquid medium required an additional process for common materials and did not utilize the merits of high temperature synthesis (Kazemizadeh et al. 2017). Therefore, the generation of CDs using laser ablation method was done in vacuum method to prevent the problems present in liquid medium. Gonçalves et al. (2010) stated that the laser ablation method was optimized with variation of distance between focusing lens and carbon target. The increased distance between focusing lens and carbon target increased the area of irradiated carbon target and influence which ultimately result to wider size dispersion. Sun et al. (2006) first reported that the produced CDs from laser ablation method in water with argon carrier gas showed bright luminescence upon attaching simple organic species. Simple organic compounds like diamine-terminated oligomeric polyethylene glycol (PEG15000N) were used before acid treated at 120 °C for 72 h. The attachment process allowed surface passivation to occur which resulted to emissive surface energy traps upon stabilization. Kazemizadeh et al. (2017) elaborated that the middle energy levels of CDs caused thermal quenching and prevented elimination of photoluminescence radiation after surface passivation. Quantum confinement of these emissive surface energy traps also played a part as the large surface-to-volume ratio was needed for brighter luminescence. Despite their attractive merits, the laser ablation method was reported to have a lower quantum yield and poor control over sizes (Namdari et al. 2017).

1.5 Bottom-Up Approaches

Alternatively, the bottom-up approaches form selected molecular precursor to “quantum-sized” particles via combustion, hydrothermal/solvothermal, and microwave irradiation methods (Xu et al. 2013). The bottom-up approaches offered great advantages as morphology and size distribution of the product can be precisely controlled for surface passivation. The selection of precursors and synthesis methods in bottom-up approaches is needed to be addressed and discussed.

1.5.1 Combustion

Nersisyan et al. (2017) defined that the combustion technique is a complex process which involves self-sustaining chemical activities with rapid release of heat in the high-temperature reaction front form. The primary factors of the combustion process application in synthesis method are heat, light, and engines. In comparison with other techniques with high temperature, combustion technique does not use external heating sources and has short reaction duration with rapid product cooling process which allowed the production of non-equilibrium products that have unique biological, electrochemical, mechanical, and physical properties. The combustion method used soot-derived sources from candles or natural gas to synthesize CDs (Baker and Baker 2010). Soot carbon was referred to as the carbonaceous aerosol dark component which consisted of pure elemental carbon with highly polymerized organic material by Cachier et al. (1989). Hossain and Islam (2013) stated that the incomplete combustion process created nonuniform size and shape CDs. Incomplete combustion typically occurs when oxygen was insufficient for complete reaction with fuel to produce water (H2O) and carbon dioxide (CO2). Incomplete combustion was utilized to produce carbon soot. An example of incomplete combustion of dodecane was shown in the equation below. In reality, actual combustion process produced a wide range of major and minor products such as carbon monoxide (CO) and pure carbon (C).

$$ {\mathrm{C}}_{12}{\mathrm{H}}_{26}+{\mathrm{O}}_2\to \mathrm{C}+{\mathrm{C}\mathrm{O}}_2+\mathrm{CO}+{\mathrm{H}}_2\mathrm{O} $$
(1.1)

This simple and effective synthesis via combustion method was reported by Tian et al. (2009). Combustion soot of natural gas produced 5 nm diameter CDs that exhibited indirect bandgap semiconductor material behavior and electrochemical activities. Another study conducted by Rahy et al. (2012) synthesized CDs by combusting an aromatic compound in a controlled environment of a Pyrex® glass container. The aromatic compounds used are benzene, toluene, and xylene. The morphology of CDs can be controlled by usage of raw materials. Kshirsagar et al. (2017) compared different organic precursor such as almond flames, burnt almond char, clarified butter, and mustard oil for the CDs synthesis. Their study discovered that the saturated fatty acids present in fats or oils contained high percentage of carbon for CDs synthesis. These findings would allow the utilization of food products into CDs synthesis via combustion technique.

1.5.2 Hydrothermal/Solvothermal

Hydrothermal/solvothermal synthesis method is the most widely used bottom-up method as it allowed formation of high pressure and temperature in an autoclave condition to obtain good crystal morphology of CDs (Xu et al. 2013). Hydrothermal method utilized water solvent, whereas solvothermal method used organic solvents such as benzene, dimethylformamide (DMF), or dimethyl sulfoxide (DMSO). These synthesis methods were also reported by Wang and Hu (2014) to be low cost, environmentally friendly, and nontoxic as their precursor used were of chemical or biological origins. Chemical materials such as citric acid and Tris were used in production of CDs as they were relatively cheap and environmentally friendly (Zhou et al. 2015). The citric acid- and Tris-derived CDs exhibited strong fluorescence upon contact with Fe3+ ions and hypotoxicity toward living cells. Other chemicals such as ethanol in hydrogen peroxide solution were able to synthesize CDs that were temperature sensitive and able to detect hypochlorite ions at high quantum yield of 38.7% without surface passivation or even heteroatom doping as described in Hu et al.’s (2016) study. The ethanol solution was used as the carbon source as it is the most common and cheap reagent in the laboratory. Similarly, nontoxic and nonimmunogenic polyethylene glycol can act as passivating solvent to produce bright, photostable, and tunable photoluminescence-emitted CDs (Fan et al. 2014). According to Sharma et al. (2017), biological materials used as green precursors for hydrothermal/solvothermal synthesis are fruits, fruit juices, fruit peels, vegetable, spices, plant leaves, beverages, animal, animal and human derivatives, and waste materials. A study by Sim et al. (2018) had reported that the CDs produced using sugarcane juices enhanced the solar photocatalytic performance of g-C3N4 upon coupling. Besides that, vegetables such as broccoli were also used to produce CDs that detected Ag+ ions, as per a study by Arumugam and Kim (2018). Bombyx mori silk that was made up of sericin and fibroin was able to produce CDs at hydrothermal condition for cell imaging application as shown in Fig. 1.3 (Wu et al. 2013). CDs can also be synthesized using waste materials like cigarette filters to detect Sudan I dye with a high selectivity and sensitivity (Anmei et al. 2018). Despite hydrothermal/solvothermal advantages, Hu et al. (2010) reported that the hydrothermal/solvothermal method required high temperature in the synthesis method which would create impracticality in large-scale production. The low-temperature hydrothermal/solvothermal method, however, required additional metal ions or templates to produce different morphologies of carbon materials for optimization of their applications.

Fig. 1.3
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Illustration of CDs formation for Bombyx mori silk. (From Wu et al. 2013)

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1.5.3 Microwave Irradiation

Guo et al. (2017) stated that microwave irradiation method is an efficient synthesis method as it is time-saving, cost-effective, and energy-conservative. Microwave irradiation is located between infrared irradiation (0.001 m) and radio waves (1 m) in the electromagnetic spectrum. Microwave irradiation had a strong penetrating ability in various mediums and caused charged particle interaction (Zhang et al. 2007). The charged particle interaction changed thermodynamics function by reducing activation energy of materials and weakening the chemical bond intensities. Materials that produce heat after interaction with microwave irradiation are known as microwave absorbers . Carbon was reported to be a good microwave absorber as it generates heat upon contact with microwave irradiation, as per a research by Menéndez et al. (2010). This synthesis method was described to have a faster reaction time as compared with conventional hydrothermal/solvothermal methods (Pham-Truong et al. 2018). L-lysine-derived CDs produced in Choi et al.’s (2017) study had strong blue photoluminescence after 5 min of microwave irradiation. The produced CDs from L-lysine can be used for biological application as it had a 97% cell viability assay with KB and MDCK cells and can be exploited in cellular imaging with successful uptake through endocytosis. In addition, Wang et al. (2011) reported that the CDs produced from graphite oxide had a higher absorption and lifetime of 3.72 ns through microwave heating route. The microwave irradiation allowed materials to grow uniformly as it was able to produce force upon charge particle to move or polarize. Subsequently, the molecular movement would cause friction and collisions and, thus, generate rapid heating which eliminates temperature gradient effect. In another study done by Yang et al. (2015a), CDs produced from folic acid and urea via microwave route had good biocompatibility and cytotoxicity as they distinguished GES-1 normal cell and HeLa cancer cell. However, the present CDs production from microwave irradiation was low as it was less than 40%, and it would significantly hinder the practical application of CDs (Wang et al. 2017a).

1.6 Structural Properties

The surface properties of green precursor-derived CDs are investigated using the X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The internal structural properties are analyzed using high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and X-ray powder diffraction (XRD).

1.6.1 Surface Properties (XPS and FTIR)

Both XPS and FTIR results can be used to confirm the functional groups in the CDs. Figure 1.4a, b shows the CDs prepared from orange peels consist of various oxygen-containing functional groups such as C=C/C–C, C–OH/C–O–C, C=O, O–C=O, C–O, C=O, and C–OH/C–O–C. The XPS results are consistent with the FTIR spectra (Fig. 1.4c) which detected the C=C stretching of polycyclic aromatic hydrocarbon and carbonyl groups (C=O). Besides, the signals arising from the furan rings of 5-hydroxymethylfurfural and nonaromatic compounds were also detected by nuclear magnetic resonance (NMR) (Prasannan and Imae 2013). For sulfur- and nitrogen-doped CDs (S–N–CDs) derived from hair fiber, Sun et al. (2013) reported the additional existence of –SO3−, C–N, C–S, N–H, and S–H bonds in FTIR spectra which is in consistence with the XPS spectrum. The presence of –C–S- covalent bond of the thiophene-S is confirmed by the peaks at 163.5 and 164.6 eV, while the pyridinic N and pyrrolic N were observed at 398.5 and 399.7 eV. The N-CDs derived from onion are highly soluble in water and able to serve as linkers for the attachment of biomolecules owing to the presence of different functional groups such as –OH, –COOH, and –NH in N-CDs. The XPS N 1 s spectrum indicated the presence of C–N–C (398.5 eV) and N–H (400.2 eV) bonds in N-CDs which was consistent with –N–H stretching vibrations (3200–3400 cm−1) and C–N stretching vibrations (1340 cm−1) in FTIR spectrum (Bandi et al. 2016).

Fig. 1.4
figure 4

XPS scans (a) C1s, (b) O1s regions of CDs, (c) FTIR spectra of CDs, and (d) 13C solid-state NMR spectrum of CDs. (From Prasannan and Imae 2013)

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1.6.2 Internal Structural Properties (HRTEM, Raman, and XRD)

As shown in Table 1.1, the particle size of CDs varies with the type of precursors used to produce the CDs, ranging from 1 to 40 nm. Yu et al. (2015) synthesized oatmeal-derived CDs with larger particle size between 20 and 40 nm with relatively good quantum efficiency of 37.4% (Fig. 1.5a, b). Most of the reported CDs exhibit a broad diffraction peak at 25° with d-spacing ranging from 0.32 to 0.34 nm which is attributed to highly disordered carbon atoms (Zhu et al. 2013; Ding et al. 2017). The shift of diffraction peak to 22.6° and the increase of d value to 0.39 nm (Fig. 1.5c) suggested the amorphous nature of CDs and the introduction of oxygen functional groups during the synthesis (Yu et al. 2015). Similar results were also reported by other researchers (Alam et al. 2015). The absence of lattices in CDs due to the amorphous nature was also reported by other researchers (Liao et al. 2016; Essner et al. 2016). Nevertheless, lattice spacing of 0.226 nm was observed in N-CDs derived from rose-heart radish, indicating the graphitic nature of N-CDs (Liu et al. 2017a). Alam et al. (2015) found that the lattice spacing of cabbage-derived CDs is 0.21 nm, which corresponds to sp2 (1120) graphitic crystal phase of graphene. CDs possess several defects which can be observed in Raman spectrum. Two obvious peaks at D band (~1360 cm−1) and G band (~1580 cm−1) (Fig. 1.5d) are attributed to the partially disordered graphite-like structure of sp3 and sp2 hybrid carbons, respectively (Li et al. 2013; Yu et al. 2015).

Fig. 1.5
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TEM image (a), size distribution histogram (b), XRD pattern (c), and Raman spectrum (d) of the CNPs. (From Yu et al. 2015)

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1.7 Other Properties

Other properties of CDs such as optical absorption, photoluminescence, upconverted photoluminescence (UCPL) properties, separation of charge carriers, and quantum yield will be discussed in following sections.

1.7.1 Optical Absorption

For coriander leaf -derived CDs solution, Sachdev and Gopinath (2015) observed two absorption peaks at 273 nm and 320 nm which were ascribed to π–π* transition of C=C bonds and n–π* transition of C=O bonds in Fig. 1.6b. Similar observation was reported by earlier researchers (Sachdev et al. 2014; Sachdev et al. 2013; Park et al. 2014). Some researchers observed a maximum absorption peak in the range of 230–278 nm with a tail extending toward the visible region in Fig. 1.7a (Wu et al. 2013; Ramanan et al. 2016; Wei et al. 2014).

Fig. 1.6
figure 6

(a) Schematic illustration depicting one-step synthesis of CDs from coriander leaves. (b) UV-vis absorption spectrum of CDs. (c) Fluorescence emission spectra of CDs at different excitation wavelengths ranging from 320 nm to 480 nm with increments of 20 nm (inset: normalized emission intensity). (From Sachdev and Gopinath 2015)

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Fig. 1.7
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Characteristic optical spectra of CDs. (a) UV-vis absorption 10 spectrum and photoluminescent spectra. Inset: photograph taken under daylight (left) and 365 nm UV light (right). (b) Emission spectra of CDs recorded for progressively longer excitation wavelength in 10 nm increment from 370 nm to 500 nm. Inset: the normalized PL emission spectra. (From Wu et al. 2013)

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1.7.2 Excitation Wavelength-Dependent Fluorescence

The fluorescence emission of CDs depends on the excitation wavelength. Most of the studies show that the maximum emission tends to shift toward higher wavelength with decreasing intensity when the excitation wavelength increases (Figs. 1.6c and 1.7b) (Sachdev and Gopinath 2015; Wu et al. 2013; Liu et al. 2012). This indicates that the emission of CDs could be tuned by altering the excitation wavelength. It was previously reported that the radiative recombinations of the surface-confined electrons and holes contributed to the fluorescence emission of CDs (Cao et al. 2012a; Sk et al. 2014; Zhang et al. 2015). The doping of CDs with nitrogen or sulfur could increase the QY fluorescence due to the formation of new surface states, which trapped the electrons and caused the radiative recombination (Dong et al. 2013). The chemical and electrical structure of CDs is proportionally dependent on the amount of nitrogen and phosphorus doping, thus increasing the fluorescence property of CDs (Zhu et al. 2013). Till now, the origin of the fluorescence of CDs is still not fully understood. Other researchers claimed that this behavior is caused by bond disorder-induced energy gaps (Luo et al. 2009), quantum confinement, and different energy levels associated with surface states arising from the presence of surface functional groups like C-O, C=O, and O=C-OH (Chandra et al. 2012; Bao et al. 2011; Sahu et al. 2012; Li et al. 2011).

1.7.3 Upconverted Photoluminescence (UCPL)

The UCPL properties arise from the multiphotons activation process and cause the simultaneous absorption of two or more photons. Such emission is called anti-Stokes photoluminescence, where the emission wavelength is shorter than the excitation wavelength (Fig. 1.8) (Wang and Hu 2014; Mehta et al. 2014). This feature renders CDs ability to absorb near-infrared light (NIR) and emit UV and visible light which is suitable for the photocatalysis application under sunlight irradiation. A recent work by Sim et al. (2018) found that the sugarcane juice-derived CDs did not exhibit detectable upconversion fluorescence when the excitation wavelength exceeded 540 nm (Fig. 1.9a). Similar results are reported by Wen et al. (2014). They suggested that the frequently obtained UCPL properties could be contributed by the normal fluorescence excited by the leaking component from the second diffraction in the monochromator of the fluorescence spectrophotometer.

Fig. 1.8
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Fluorescence emission spectra of CDs obtained at different excitation wavelengths progressively increasing from 300 nm to 540 nm with a 20 nm increment. (From Mehta et al. 2015)

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Fig. 1.9
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(a) The photoluminescence (PL) emission spectrum of CDs at different excitation wavelengths and (b) PL spectra of the composites. (From Sim et al. 2018)

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1.7.4 Electron Transfer Property

Sim et al. (2018) reported that the incorporation of sugarcane juice-derived CDs into g-C3N4 promoted the efficient separation of electron and hole pairs. However, the separation efficiency dropped if the concentration of CDs was excessive (Fig. 1.9b). Their work also mentioned that the π-conjugated CDs act as a photosensitizer to sensitize g-C3N4 and donate electrons to g-C3N4 (Fig. 1.10). The electron transfer property of CDs enables their wide applications in metal ion detection. According to Roshni and Praveen (2017), the electron or energy transfer from CDs surface groups to metal ions like Hg (III) significantly quenched the fluorescence emission of CDs. Such electron transfer process is induced by the presence of hydroxyl and carboxyl or carbonyl groups on the surface of CDs, resulting in the strong binding interaction between the metal ion and these functional groups (Roshni and Praveen 2017; Zhou et al. 2012).

Fig. 1.10
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Electron transfer mechanism of CDs/g-C3N4. (From Sim et al. 2018)

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1.8 Applications

CDs have obviously emerged to become an important research field that is still experiencing rapid advances, as reflected by the large number of recent publications. The CDs possess unique features such as tunable fluorescence, easy functionalization, high aqueous solubility, and excellent optical and biological properties which enable their wide applications in different fields. This section summarizes the application of CDs in bioimaging, sensing, photocatalysis, drug delivery, and gene transfer.

1.8.1 Bioimaging

Martynenko et al. (2017) defined that the bioimaging process occurred with labeling of target biomolecule with contrast agent via specific biochemical techniques for read-out system detection. An ideal contrast agent is soluble and stable in suitable buffers or biomatrices and has good analytical signal and photophysics data with easy application in practical bioimaging process. CDs were reported to be able to fulfill the requirements as an ideal contrast agent due to their excellent photoluminescence property and biocompatibility to target biomolecule (Peng et al. 2017). The distinctive advantages of CDs in bioimaging such as excellent photostability, multicolor emission profile, and small size were also described by Zhang and Yu (2015). Fluorescent CDs derived from 2-propanol in Angamuthu et al.’s (2018) study showed clear zone of HeLa cell nuclei upon excitation of 488 nm wavelength in confocal microscopy imaging. The 2-propanol-derived CDs also exhibited low cytotoxicity and positive biocompatibility upon HeLa cell incubation which conformed to the advantages of CDs. Another research by Lin et al. (2014) demonstrated that the CDs synthesized from shrimp eggs exhibited blue, green, and red fluorescences at cell membrane and cytoplasm of SK-Hep-1 cells upon delivery for 24 h, shown in Fig. 1.11. The multicolor emission of CDs performed better in bioimaging application compared with traditional fluorescent protein with single fluorescence characteristic. Yang et al.’s (2009) study was able to develop highly fluorescent CDs as prominent contrast agent for optical imaging in vivo for the first time. The PEGylated CDs was injected subcutaneously into female mice and imagined in Lumazone FA in vivo imaging system with 470 nm and 525 nm emission filters. Figure 1.12 shows the intravenous injection of CDs into the mice. It was demonstrated that the fluorescence from CDs was readily detected in urine which suggested urine excretion of CDs from the mice body after 3 h postinjection. This deduced that the CDs would be excreted after injection which reduced its toxicity in the mice. Furthermore, Cao et al. (2012) also confirmed Yang et al. (2009) report of applied CDs in mice for bioimaging. They presented that fluorescence signals of CDs injected in subcutaneous layer of female mice would diffuse and fade slowly within 24 h. The CDs also migrated along the arm of mice with consistent fluorescent emission. Cao et al. (2012) also compared with conventional CdSe/ZnS QDs in bioimaging of mice and discovered that the resulting fluorescence images of both CDs and CdSe/ZnS QDs were similar in brightness and fluorescent quantum yields. However, CDs had a better advantage compared with CdSe/ZnS QDs as it is nontoxic to the mice.

Fig. 1.11
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Images of bright-field and fluorescence images of SK-Hep-1 cells treated with CDs derived from shrimp eggs. (From Lin et al. 2014)

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Fig. 1.12
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Intravenous injection of (a) bright field, (b) as-detected fluorescence (Bl (bladder) and Ur (urine)), and (c) color-coded images. Similar order for the images of the dissected kidneys at the lower left and liver at lower right. (From Yang et al. 2009)

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1.8.2 Sensing

Han et al. (2017) stated that the sensing based on fluorescence had received huge attention due to its excellent sensitivity, low cost, and short response time. The fluorescence sensing mechanism involved four different mechanisms such as photoinduced electron transport (PET), resonance energy transfer (RET), photoinduced charge transfer (PCT), and inner filter effect (IFE) (Sun and Lei 2017). The PET process involved the internal redox reaction, while RET used long-range dipolar interaction between excited state of CDs and another species to produce fluorescence emission. In addition, PCT mechanism transfer electrons from electron donor to acceptor in partial charge transfer of fully conjugated π system. The IFE process differed from the rest of the mechanism as this mechanism utilized quenching, which allowed fluorescence lifetime to be independent of total intensity. CDs can be used as an effective sensing agent as it is abundant, benign, and low cost. In comparison with other traditional semiconductor QDs, CDs had higher solubility, chemical inertness, resistance toward photobleaching, and facile modification. Han et al. (2018) described that the synthesized CDs with various functional group were able to detect multiple compounds such as hydrogen peroxide, glutathione, mercury ion (Hg2+), silver ion (Ag+), tetracycline, and enrofloxacin hydrochloride. Furthermore, the fungus-derived CDs were able to recover 105.0% of hyaluronidase in human urine (Yang et al. 2017). Hyalurondase is an enzyme that degraded hyaluronic acid and had been recognized as a new tumor marker type which allowed the fungus-derived CDs to detect tumor in human body. Li et al. (2017) also reported hydrothermally synthesized CDs were able to detect iron ion (Fe3+), chromium (Cr(VI)) ion, and ascorbic acid in lake water and industrial water via PET, IFE, and elimination IFE mechanisms. These mechanisms were shown in Fig. 1.13. Fluorescent CDs produced from carnation in Zhong et al. (2016) exhibited colorimetric response from pH ranging from 12.8 to 4.6 and under different light sources of daylight and UV light. The fluorescent CDs that corresponded to the different pH values showed that CDs act as a colorimetric and fluorescent sensor for pH sensing. Hematin that bound to the membrane of red cell and caused any related sickle-cell anemia was sensed by p-aminobenzoic acid-synthesized CDs in human blood sample using IFE (Zhang et al. 2018). Therefore, the CDs were able to distinguish hematin from complex solution and act as good and stable as fluorescence nanoprobe.

Fig. 1.13
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Hydrothermally synthesized CDS in three sensing modes for Fe3+, Cr (VI), and ascorbic acid via different methods including PET, IFE, and elimination IFE mechanism type. (From Li et al. 2017)

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1.8.3 Biomedicine (Drug Delivery and Gene Transfer)

The fluorescence property of CDs had allowed the application of biomedicine in targeted drug delivery and gene therapy owing to their aqueous solubility, facile synthesis, and quantum yield (Jaleel and Pramod 2017). Drug delivery significantly determined the therapeutic efficiency and nonproductive distribution of specific treatments. Yuan et al. (2017) detailed that the chemotherapeutics with reduced drug delivery can lead to increase drug dosages and undesirable side effects during treatment. CDs were able to aid the chemotherapeutic treatment as they exhibit superb loading, release, target, and tracking of drug properties. Carrot root-derived CDs were reported by D’souza et al. (2018) to effectively hold and deliver mitomycin drug to cancer cell due to their biocompatibility and ultrasmall size. The green-synthesized CDs acted as optimal drug vehicles by offering excellent selectivity and specificity for mitomycin drug delivery system development. Feng et al. (2016) described that the modification of single surface property of CDs formed charge-convertible property in mildly acidic tumor extracellular microenvironment at approximately pH 6.8. The CDs with the addition of polyethylene glycol allowed pH-responsive characteristics by changing the charge to negative at normal physiological environment at pH 7.4 and reverse to positive charge at tumor extracellular microenvironment. These properties allowed the CDs to have a prolonged blood circulation time, enhanced permeability and retention (EPR) effect, enhanced tumor cell internalization, less side effects at normal physiological condition, improved endosome escape, and controlled cytotoxic release of cisplatin, an anticancer drug in cancer cell as shown in Fig. 1.14.

Fig. 1.14
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Illustration for the drug delivery process of CDs-modified polyethylene glycol: (1) prolonged blood circulation time, (2) enhanced permeability and retention (EPR) effect, (3) enhanced tumor cell internalization, (4) less side effects at normal physiological condition, (5) improved endosome escape, and (6) controlled cytotoxic release of cisplatin, an anticancer drug in cancer cell. (From Feng et al. 2016)

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Alternatively, the high photoluminescence property allowed the CDs to act as a promising probe for gene therapy. The gene therapy developed gene carriers which were initially done via virus. Viruses were effective transfection vectors but had issues with carcinogenicity, immunogenicity, and ammation. Therefore, CDs were developed to overcome virus limitation as CDs had high transfection efficiency and low toxicity (Yang et al. 2017). Sodium alginate-derived CDs were synthesized and acted as a gene vector for plasmid TGF-β1 delivery in Zhou et al.’s (2016) study. The green CDs were compared with Lipofectamine 2000, branched PEI, and semiconductor QDs in transfection efficiency, and the results showed that the CDs had a higher transfection efficiency due to its low toxicity, small size, positive surface charge, and strong condensation effect on pDNA macromolecule that prevented pDNA enzymolysis. Zhang et al. (2016) stated that the fluorescent CDs with small interfering RNA (siRNA) conjugate were able to deliver 1/30 of siRNA amount in comparative with gold nanoparticles. The fluorescent CDs-based siRNA conjugate reduced approximately 80% of A375 cells via cellular Plk1 mRNA treatment which led to induce apoptosis in 31.90% of A375 cells and 20.33% of MCF-7 cells. These apoptosis induction efficiency was higher than commercially available Lipofectamine 200, which further proved the efficiency of CDs-based siRNA conjugate in gene delivery and therapy.

1.8.4 Photocatalysis

Han et al. (2018) described that photocatalysis involved the synergistic effect of light and catalyst during energy conversion material process which degraded contaminates and obtained hydrogen energy. An ideal photocatalyst for photocatalysis process should have a wide light absorption range and high photogenerated charge carrier separation efficiency for enhanced overall quantum efficiency and practical applications. Photocatalysis process can be utilized in water splitting technologies which generate hydrogen generation and pollutant removal remediation methods. CDs were able to produce photogenerated charge carriers under light irradiation and had unique properties of low cost, excellent photoelectric properties, enhanced adsorption capacity, and nontoxic nature. Besides that, CDs were reported by Yu et al. (2016) to be a viable alternative in comparison to the expensive platinum group metal-modified photocatalyst as CDs had an intrinsic bandgap owing to quantum confinement and edge effect which enhanced the separation of photogenerated charge carrier (Lu et al. 2016). As reported by Yang et al. (2015), pure CDs produced without any modification and co-catalyst generated hydrogen from pure water at a higher rate of 34.8 times compared with commercial TiO2 Degussa P25 when methanol was utilized as the sacrificial reagent. The enhanced photocatalytic ability of pure CDs in water splitting was due to CDs big band gap, low photoluminescence intensity, strong oxidation and reduction potential which ultimately, lowered the recombination rate of photogenerated charge carriers and increased the photocatalytic activities. For optimization of photocatalytic process, the CDs were required to adopt two material design approaches such as the band alignment and interfacial properties manipulation and properties tuning on heteroatom doping, particle size and surface state. The first approach of band alignment manipulation was difficult to be achieved singly as the highest unoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was governed by size and shape factors. Hu et al. (2016) explained that the doping of heteroatom in CDs was able to tailor the charge distribution of CDs surface which facilitate better photogenerated charge carrier separation. The synthesized chlorine (Cl) and phosphorus (P) functionalized CDs showed higher photocatalytic efficiency in methylene blue and methyl orange dye under visible light irradiation. Another study done by Sim et al. (2018) showed that the CDs and graphitic carbon nitride (g-C3N4) composite completely degraded Bisphenol A (BPA) pollutant after 90 min under solar irradiation due to its superb visible light absorption capability. The approach of CDs application in photocatalytic treatment of recalcitrant pollutant would prove to be a renewable and sustainable method for future treatments.