Fluorescence “Giant” Red Edge Effect
4.2 Physical properties
parameters. CQDs are generally quasi-spherical in shape, and depending on the pre- cursors and synthetic pathways utilized, they can be crystalline or amorphous car- bon with a variety of adjustable surface groups[6]. TEM and SEM can be used to analyze the morphology, size distribution, and particle size of CQDs. If the particle size varies from 1 to 20 nm, SEM is used; however, if the measurement surpasses the resolution of SEM, TEM can provide greater resolving power[10]. CQDs uni- form dispersion and spherical shape with its agglomeration are analyzed by TEM, and the lateral size distribution in particle size and lattice fringes may measure by HRTEM. The uniformity, 2D and 3D topographic structure can be observed by atomic force microscopy (AFM). The peak width decreases as the relative intensity of the XRD peak increases, indicating that CQDs carbonation and crystallinity are improving[25]. Thus, XRD technique provides a valuable information on the parti- cle size, crystal structure, and purity of CQDs[21]. CQDs are primarily made up of carbon atoms, with oxygen, nitrogen, and maybe other dopants added depending on the elements present as well as reaction circumstances such as time and tempera- ture. Depending on which materials are integrated into the carbon network and sur- face state, the actual structure of spherical CQDs might vary in complexity.
Moreover, the parameters of the particle formation are determined directly by its various heating methods[23]. The chemical bonds and compositions of CQDs were revealed using the XPS and FTIR methods. XPS can identify the C, O, and N ele- ments present in CQDs, while FTIR can see functional groups such as carbonyl, hydroxyl, carboxyl, amide, and amino groups on the surface of CQDs [25].
Mechanical qualities of CQDs include elongation at break, tensile strength, scratch hardness, toughness, flexibility, and impact resistance[26]. Elasticity (or flexibility) of CQDs was examined at several pH levels, including cycling between acidic and basic states, and CQDs were shown to be able to keep their structure and fluores- cence ability when exposed to an excitation source[27]. CQDs have an aromatic carbonized structure that increases their strength, and their peripheral polar func- tional groups generate strong physiochemical interactions that boost their elasticity and toughness[26]. The surface-functionalized CQDs exhibit variable PL behavior in response to changes in their surroundings, allowing them to operate as sensors for monitoring changes in pH or temperature within and around the defined target.
The sensitivity to pH was attributed to hydrogen bonding between the surrounding ions and the surface functional groups of CQDs[27]. The density of CQDs can be observed as 1.0032 g/mL. CQDs whose density may be greatly boosted by co- doped sulfur atoms provide important scientific insights into CQD fluorescence enhancement mechanisms[28]. CQDs possess high purity and good water solubility [29]. Many carboxyl moieties present on CQDs surface impart excellent solubility in water. Citric acid is an important organic acid with strong water solubility that is often employed as a carbon source to synthesize CQDs[2931].
Generally, CQDs possess tunable emission fluorescence in 450550 nm range.
CQDs are chemically modified and have their surfaces passivated using a variety of organic, polymeric, inorganic, and biological materials. The fluorescence as well as the physical features of the surface were improved by this passivation. When aque- ous solutions of N-CQDs are excited at 360 nm, CQDs show a bright luminescence
centered at 450 nm [25]. CQDs exhibit ferromagnetism property, that is, CQDs have a substantial, positive susceptibility to an externally applied magnetic field, as well as a strong attraction to it and the ability to maintain their magnetic character- istics when the external field is withdrawn[26]. The magnetic result implies all the spins contribute to the magnetism, including those at the particle surface [26,32].
Fluorescent CQDs in aqueous media are mostly transparent in appearance and emit color under UV light. It is also observed that at elevated temperatures the CQD solutions turns to yellow color and on cooling to room temperature again it turns to transparent color [33,34]. CQDs possess good electrical conductivity and it is enhanced by doping with N, S, metals, and conducting polymers. The electrical conductivity and current density of the N-CQDsgraphene oxide (GO) hybrid cata- lyst is significantly improved due to the well-connected junctions of GO and N- CQDs, which allow electrons to be readily and swiftly transported. The edge-filled element dopant in the hybrid catalyst leads to a significant increase in electrical conductivity as well as large surface area when compared to standard PtC cata- lysts [34]. The electrical features of CQDs include a significant inherent dipole moment and huge bandgap. These can efficiently collect visible light energy, gener- ate electronhole pairs, rapidly separate electrons and holes, and minimize elec- tronic loss, allowing them to quickly and effectively execute optically electrical impulses, and also electrical impulse transformation and transmission. These excel- lent electrical characteristics benefit logic gates, transistors, solar cells, ultrafast optical switches, quantum computing, etc.[35]. CQDs have excellent thermal sta- bility and can withstand temperatures of up to 800C, enabling for a wide range of applications in high-temperature conditions[36]. Javed et al. calculated thermal sta- bility of CQDs up to 1000C [37]. Furthermore, CQDs aromatic carbonaceous structure and peripheral polar hydroxyl groups cause substantial physiochemical interactions with the matrix, which improves heat stability[38]. The functionalities allow the surfaces of CQDs to take on either a hydrophilic or hydrophobic charac- ter, resulting in the required thermodynamic stability in various solvents [17].
CQDs show good ionic stability in a high ionic strength solution[30]. CQDs exhibit high chemical stability varied by physical and chemical modulations [31].
Mansuriya et al. highlighted the long-term chemical stability of CQDs owing to their ability to connect strongly with catalysts via electrostatic interactions[39].
4.2.1 Physiochemical properties (catalytic)
The conductivity of the CQDs mostly depends on the size, shape, heteroatom dop- ing, and changes in surface functional groups which possess a large surface area and quick charge transfer phenomena. Hence it can be applicable for the photo (electro)catalytic applications. Organic groups facilitate water molecule adsorption and improve coordinating sites with metal ions, resulting in the formation of CQD- hybridized catalyst. CQDs may be employed as efficient confined electron accep- tors in metal organic frameworks to isolate photogenerated carriers from core metallic nanoparticles, boosting their photocatalytic activity significantly[22]. The demand for clean, long-lasting energy has spawned a burgeoning branch of
chemical study. Photocatalysts based on CQDs can be driven by sunlight to effi- ciently push forward the chemical processes required to degrade toxic organic dyes and pollutants into smaller, more ecologically friendly molecules, or to photosplit water to generate hydrogen for clean energy production[27](Fig. 4.10)[40].
Heteroatoms, such as N-CQDs, S-CQDs, and P-CQDs, play vital role by elec- tronic structures in engineering field as well as act as reactive catalytic sites during the electrocatalytic process. This physiochemical reaction can be classified into three divisions as shown inFig. 4.11AandFig. 4.12.
Carbon-based materials, especially CQDs, have gained a great deal of interest in the fields of energy storage and conversion as a result of emerging environmental concerns.
The functional groups (e.g.,OH,COOH,NH2) on the surface of CQDs can be utilized as active centers for transition metal ions. Enhanced electron transport through internal interactions might increase the electrocatalytic efficiency of heteroatom-doped CQDs with several components. CQDs hybridized with other inorganic materials, such as metal sulfides, layered double hydroxides (LDHs), and metal phosphides, can be uti- lized as excellent electrocatalytic materials for oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and carbon dioxide reduction reaction (CO2RR)[41]. Mostly reported carbon-based materials usu- ally respond to ORR, meanwhile doping CQDs with other binary/ternary metal endowed with transition metal dichalcogenides enhances their HER and OER activity.
Figs. 4.10, 4.11B, and 4.12show photocatalytic reactions, types of catalytic reactions, and various applications of composite CQDs, respectively.
Figure 4.10 Photocatalytic mechanism of CQDs[40].CQDs, Carbon quantum dots.
Source: Reproduced with permission from Z. Zhang, T. Zheng, X. Li, J. Xu, H. Zeng, Progress of carbon quantum dots in photocatalysis applications,Particle and particle systems characterization, vol. 33, no. 8, Wiley-VCH Verlag, 2016, pp. 457472, doi:10.1002/
ppsc.201500243.
4.2.2 Optical properties
CQDs show some of the significant optical properties, such as absorption, PL, fluo- rescence, etc., which are mostly depend on their size and the surface properties. We can control the particle size, shape, and other parameters in different synthesis routes. Fig. 4.13A shows fluorescence of multicolor bandgap fluorescent CQDs Figure 4.11 (A) Different catalytic reactions of CQDs. (B) schematic illustration of the photocatalytic degradation of organic pollutants using CQDs as photocatalyst.CQDs, Carbon quantum dots.
Source: Reproduced with permission from U. Abd, L. Yong, C. Yin, E. Mahmoudi, A review of carbon quantum dots and their applications in wastewater treatment, Advances in Colloid and Interface Science 278 (2020) 102124, doi:10.1016/j.cis.2020.102124.
Figure 4.12 Composite and hybrid structures of CQDs showing different applications.
CQDs, Carbon quantum dots.
synthesized through hydrothermal method at different time periods that showed dif- ferent colors from blue to red under UV light. Fig. 4.13B shows phenomena of enhanced electronhole pairs. In further sections, we have elaborately explained optical properties such as absorption, PL, fluorescence, etc. (Fig. 4.14)[42].
4.2.2.1 Absorption
UVvisible spectrophotometer is generally used to measure the CQDs capacity to absorb light in the UVvisible region, with the highest absorption peak occurring at 230 nm and a tail beginning atB300 nm that involves electron lone pairs. The absorp- tion peak position can be influenced by oxygen in CQDs, and by doping[45]. CQDs possess optical absorption peaks caused by the n transition of sp2-conjugated carbon as well as the n- transition of hybridization in the UVvisible region. Heteroatoms such as N, S, and P hybridized might be accountable for the optical absorption peaks of CQDs, and optical absorbance is influenced by the surface passivation or modifica- tion procedure. CQDs can be produced as red, green, or blue luminous. These CQDs absorption transitions observed as redshifted, owing to the reduced form of electronic Figure 4.13 (A) Optical properties of MCBF-CQDs exhibiting various colors under UV light[43]. (B) Enhanced electronhole pairs (excitons) multiplication in CQDs[44].CQDs, Carbon quantum dots.
Source: From with permission (A) T. Yuan, et al., “Carbon quantum dots: an emerging material for optoelectronic applications,” Journal of Materials Chemistry C 7 (23) (2019) 68206835, doi:10.1039/c9tc01730e; (B) A. Luque, A. Martı´, A. J. Nozik, “Solar Cells Based on Quantum Dots : Multiple Exciton Generation and Intermediate Bands,” vol. 32, Cambridge University Press, 2007, pp. 236241.
band gaps compared to others[46]. The absorption band was characteristic of an aro- matic and it correlated to the pp transition of carbon bonds C5C, with faint peak at 310 nm owing to the pp transition of C5O[47].
4.2.2.2 Photoluminescence
PL plays a vital role which can be manipulated by used precursors along with various synthesis methods. There are several reports on CQDs showing variety of PL wave- lengths ranging from visible to near-infrared (NIR). CQDs have unique emission wave- length and intensity as distinctive properties, depending on the size and the surface functionality[17]. Mostly, the CQDs exhibit a wide and excitation-dependent PL emis- sion spectrum that can be easily tuned by varying the concentration, where the PL strength of the CQDs increases at first and then decreases gradually as the concentration increases. The PL emission wavelength is mostly greater than that of the excitation wave- length[41]. Many researchers demonstrated that PL is strong when the size of the CQDs are well controlled as well as when CQDs are well passivated[48].Fig. 4.15Aillustrates the spectral response of CQDs with glucose/NaOH, whereasFig. 4.15Bdepicts the spec- tral response of CQDs with glucose/HCl. Under UV and visible excitation, the respective visible emission of CQDs is across the blue-to-red range of wavelength[49].
Generally, PL can be further classified into two types: fluorescence and phosphorescence.
Figure 4.14 Classification of luminescence and their energy sources.
Source: Edinburgh Instruments, Fluorescence, phosphorescence, photoluminescence differences. Photoluminescence Differences. 2012, [Online]. Available:https://www.edinst.
com/blog/photoluminescence-differences/.
Fluorescence
G Bandgap transitions attributed by conjugated p-domains.
G Surface defects.
Generally, p-domains rich in p-electrons which are involved in sp2hybridization are responsible for fluorescence in CQDs. The fluorescence mechanism caused by CQDs surface-related defects are based on the ratio of the surface to volume, that is, smaller sizes attribute to a large surface-to-volume ratio, which attribute to a greater number of surface defects that result in enhanced fluorescence. In another approach, by passivation of the surface functional groupsCOOH and epoxy of the CQDs byCONHR and CNHR with alkylamines, the surface defect-related green emission disappears and the intrinsic blue emission is observed to be greatly enhanced [43]. Surface defects such as oxygen-containing functional groups, as well as sp3- and sp2-hybridized car- bons, can act as excitons capture centers, resulting in surface-related defect state fluo- rescence. According to density-functional theory simulations, the carboxyl groups on the sp2-hybridized carbons can cause considerable local distortions and minimize the energy gap. After the reduction of these oxygen-containing functional groups, the opti- cal characteristics may be entirely altered, such as drastically different fluorescence emission bands and intensity distributions. Red-shift of fluorescence peak positions appear with the increase of the excitation wavelength[38].
Phosphorescence
CQDs have demonstrated the intriguing ultra-long lifetime and room temperature fluorescence. The researchers employed phosphoric acid in combination with etha- nolamine or EDA and observed phosphorescence for up to 10 s. Mansuriya et al.
measured the lifespan of CQDs phosphorescence using ethanolamine and EDA to be 1.46 and 1.39 s, respectively[39]. The phosphorescence was quenched in solu- tion and only detected in the solid form and this was also ascribed to the phospho- rous doping into the CQDs structure, which created the extended lifespan emission.
The researchers also exhibited phosphorescence in CQDs without phosphorous dop- ing; these CQDs phosphorescence were with a lifespan of 1045 ms (glucose, Figure 4.15 (A) CQDs prepared from glucose/NaOH, and (B) CQDs prepared from glucose/
HCl[49].CQDs, Carbon quantum dots.
trihydrofluoride, and trimethylamine) and 747 ms (glucose, trihydrofluoride, and trimethylamine) (glucose and aspartic acid)[50]. Huang et al. also demonstrated the preparation of CQDs from citric acid and urea, then passivating them using cyanu- ric acid. Furthermore, in the phosphorescence of CQDs, hydrogen bonding plays a vital role[40]. Interestingly, CQDs displayed phosphorescence in an aqueous solu- tion with a lifespan of 687 ms, which is significantly longer than the solid powder which is around 253 ms. Increased stiffness introduced by hydrogen bonding of water and cyanuric acid of CQDs increases the phosphorescence lifespan in the aqueous solution [51]. Lim et. al. reported that the synthesis of CQDs using folic and citric acids exhibited excellent phosphorescence lifespan of about 705 ms at pH 11.5[52]. The phosphorescence was improved by increasing the conjugation of the electrons and de-protonating the carboxylic groups at a basic pH. The lifespan and QY of phosphorescence were considerably boosted as a result of this, in addition to the intraparticle hydrogen bonding[6].Fig. 4.16shows wavelength versus intensity plot for phosphorescence and fluorescence of CQDs.
4.2.2.3 Electroluminescence
Because semiconductor nanocrystals are well known for exhibiting electrolumines- cence (EL), it is no surprise that CQDs have sparked a flurry of interest in EL research that might be useful in electrochemical domains. The emission color of CQD-based light-emitting diodes may be adjusted by the driving current. Under varying working voltages, watchable EL colors detected from the identical CQDs range from blue to white[53]. The researchers developed two theories based on the conjugated p-domain bandgap emission and the edge effect generated by another surface defect to better explain the luminescence mechanism of CQDs [25]. The quantum confinement effect (QCE) of p-conjugated electrons in the sp2 atomic Figure 4.16 Phosphorescence of CQDs analyzed by wavelength versus intensity plot[50].
CQDs, Carbon quantum dots.
framework determines the EL properties of the fluorescence emission of CQDs from the conjugated p-domain, which may be changed by changing the size, shape, and edge configuration. Surface flaws such as sp2 and sp3-hybridized carbon, as well as other surface defects of CQDs, cause fluorescence emission, and even fluo- rescence intensity and peak position are connected to this defect[52,54].
4.2.2.4 Up-converted photoluminescence
In addition to traditional PL emission, in recent investigations, several CQDs have demonstrated up-converted PL (UCPL) emission. In contrast to regular PL, in UCPL the emission wavelength is shorter than the excitation wavelength. To explain such UCPL properties in CQDs, there are two kinds of mechanisms proposed by several researchers such as multiphoton active process and anti-Stokes PL. Researchers discov- ered that CQDs made by laser ablation had significant luminescence in the NIR (800 nm) with two-photon excitation, implying that they have UCPL characteristics [33]. In addition to their PL emission, Liu et al. analyzed that CQDs possess UCPL characteristics where the emission is in the range of 450750 nm, where they are excited by a long-wavelength light from 700 to 1000 nm as shown inFig. 4.17 [49].
The UCPL of CQDs can be attributed to the multiphoton activation process; this fea- ture of CQDs can be used for a variety of applications, including in vivo bioimaging, because bioimaging at longer wavelengths is usually preferred due to improved photon tissue penetration and lower background auto fluorescence[17].
Figure 4.17 Up-converted photoluminescence (UCPL) spectra of CQDs at different excitation wavelengths.CQDs, Carbon quantum dots.
Source: Adapted with permission from H. Li, et al., One-step ultrasonic synthesis of water- soluble carbon nanoparticles with excellent photoluminescent properties, Carbon 49 (2) (2011) 605609, doi:10.1016/j.carbon.2010.10.004.
4.2.3 Photoinduced electron transfer
CQDs could be quenched efficiently by either electron acceptors or electron donors in the solution[46]. Despite the fact that this light-induced electron transfer property of CQDs has lately been extensively reported, direct evidence and the essence of pho- toinduced charge separation in CQDs have yet to be achieved. Through redox reac- tions, some indirect experimental proof was gained. PL decay experiments of CQDs using the known electron acceptor 2,4-dinitrotoluene (20.9 V vs NHE) and electron donor N, N-diethylaniline corroborated this feature (DEA, 0.88 V vs NHE) [17].
Photoinduced electron transfer capabilities of CQDs offer up new possibilities for their possible applications. Irradiating a CQD solution with a noble metal (e.g., silver, gold, or platinum) salt causes the noble metal to form and deposit on the surface of the CQDs. Because the noble metal has a high electron affinity, it absorbs electrons from the connected CQDs, interrupting radiative recombination once more, resulting in the very effective static suppression of fluorescence emissions seen[17].
4.2.4 Biological properties
One of the most essential aspects of CQDs is their biocompatibility, which may be achieved in the field of bioimaging due to their low toxicity and luminous qualities in living systems. CQDs that are made by decomposing citric acid monohydrate and diethylene glycol bisphosphate have high fluorescence characteristics and minimal cytoxicity during imaging (3-aminopropyl)[52]. These CQDs are considered as impor- tant for their application in cell labeling and imaging. Herein, we investigate the cyto- toxicity of CQDs at the diverse time and concentrations[55]. The biocompatibility of CQDs, along with the wide range of surface functionalization methods, has opened up new possibilities for gene delivery. CQDs perform better than organic dyes because of their reduced cytotoxicity[26]. Metal-sensing capabilities of CQDs have been demon- strated with improved selectivity and sensitivity. Diagnostics is another major use of CQDs in biomedicine, and semiconductive quantum dots have been employed for in vivo disease diagnosis[16]. Bioimaging of HeLa cells is shown inFig. 4.18.
Figure 4.18 Bioimaging of HeLa cells. (A) Excitation at 405 nm causes blue emission. (B) Excitation at 458 nm produces green emission. (C) Excitation at 514 nm causes red emission.
Source: Adapted with permission from B. D. Mansuriya, Z. Altintas, Carbon dots:
classification, properties, synthesis, characterization, and applications in health care-an updated review (20182021), Nanomaterials 11 (2021) 10, doi:10.3390/nano11102525.