Solvatochromic Behavior of Solid-State Nitrogen-Doped Carbon Nanoparticles: Initial Study

Carbon nanoparticles (CNPs) have garnered significant attention among researchers due to their cost-effectiveness and tunable properties, rendering them applicable in a wide array of fields, including biomedicine, optoelectronics, catalysis, and sensing. The solvatochromic effect denotes the phenomenon of a compound exhibiting a color change or a shift in its photoluminescence spectrum. In this study, we investigated the solvatochromic behavior of CNPs doped Nitrogen synthesized through the solid-state route using citric acid and urea, dissolved in solvents with varying polarity indices. Significant differences in emission were analyzed using a photoluminescence (PL) spectrometer. The results exhibit the emission shifting towards longer wavelengths (red region) as the solvent’s polarity index increased. This study of solvatochromic behavior in CNPs holds substantial significance for further research into their application as sensors.


Introduction
Fluorescent carbon nanoparticles (CNPs) have attracted significant attention in multidisciplinary fields due to their distinctive advantages of tunable emission, low toxicity and abundant raw materials.The CNPs have evolved into a focal point of interdisciplinary research owing to their tunable properties and applicability across a spectrum of fields, including biomedicine, optoelectronics, catalysis, and sensing [1].To date, several experimental procedures have been conducted to shift the absorbance feature of the CNPs into the first NIR window [2].
One fascinating phenomenon within this nanomaterial landscape is the solvatochromic effect, a captivating optical behavior characterized by altering a compound's color or shifting its photoluminescence spectrum in response to variations in its surrounding solvent environment [3,4].To advance our understanding of CNPs, this study embarks on a pioneering investigation into the solvatochromic behavior of nitrogen-doped CNPs synthesized through a robust solid-state route employing citric acid and urea as precursors [5].We delve into uncharted territory by dissolving these CNPs in solvents characterized by varying polarity indices, probing their intriguing photophysical responses with the precision of a state-of-the-art photoluminescence (PL) spectrometer [6].The revelations of our study showcase a compelling trend, as CNPs demonstrate a remarkable shift towards longer wavelengths, primarily residing in the red region of the electromagnetic spectrum, with an increase in the polarity index of the solvent [4].This not only unravels the intricate interplay between CNPs morphology and solvent interactions but also unveils a pivotal opportunity for leveraging their solvatochromic attributes in novel sensor applications.

Experimental method
The citric acid (CA, 0.03 g) and urea (3,03 g) were dissolved in 10 mL of distilled water and stirred at 500 rpm until homogenous solution.The solution was transferred to the glass and evaporated by the oven at 100℃ for one hour.Then, the white powder was obtained which will be used as a precursor to synthesize the carbon nanoparticles (CNPs).Subsequently, the powder was subjected to a carbonization procedure using a domestic microwave (700W, for 3 minutes).After cooling down to room temperature (25℃) naturally, the yellow solid-state as a final product (CNPs) was achieved and grounded using agate mortar.To prepare the CNPs solution for further analysis, 0.5 g of powder CNPs were accurately dissolved in 30 mL of dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), formamide and water, based on index polarity variation, as shown in figure 1.

Results and Discussion
The synthesis process and solvatochromic behavior of CNPs have been performed using the domestic microwave radiation method, as depicted in figure 1.The process begins with homogeneous dissolution of CA and urea in water and evaporates in the oven to obtain the product as a bonded precursor.Microwave irradiation is an efficient and low-cost method because it encourages carbonization processing with short-time heating.After 3 minutes of heating with a domestic microwave (700W), the white powder will be changed to yellow powder (solid-state CNPs) which is able to fluorescent under UV light.To perform the solvatochromic behavior, the solid-state CNPs were dissolved in solvent variation based on the index polarity, such us water, DMSO, DMF and formamide [4,7].The result show that the different color (blue to light green) of the CNPs solution was achieved under UV light with an excitation wavelength of 365 nm.The prominent peaks in the CNPs absorption spectrum are in the wavelength range of 200 -300 nm, as shown in Figure .2a.The peak indicates the presence of π-π* electron transitions (in sp 2 hybridization carbon core) [8].In the wavelength range of 300 -450 nm, the second and third absorption peaks are formed involved in electron transitions from n-π* (corresponding to C-N, C-H, and another functional group) [9]. Figure .2cshows CNPs exhibit independent fluorescence at all excitation wavelengths [10].The presence of independent CNPs samples at all excitation wavelengths.The number of bonds formed in CNPs is presented in the form of an FTIR spectrum (Figure . 2b).The formed bonds include O-H at wavenumbers around 3420 cm⁻¹ and 1150 cm⁻¹, at the wavenumber of 3170 cm⁻¹ suggesting the presence of C-H sp² bonds [11], C=C bond at the wavenumber of 1660 cm⁻¹ indicates the formation of π-conjugated core structures [12].The presence of the C-N bond (1500 cm⁻¹) [13] at 1720 cm⁻¹, indicates the presence of C=O (carbonyl) groups.The formation of the N-H bond (3200 cm⁻¹) means that CNPs are hydrophilic.Lastly, the C-H sp³ bond at the wavenumber of 3060 cm⁻¹ suggests the presence of carbonization [14].Figure . 2d shows the TEM (Transmission Electron Microscopy) analysis results, indicating that the particles in this sample have a spherical shape.This analysis discovered that the most dominant particle size is approximately 50 -60 nm.The varying colors observed in the emission spectrum of CNPs after being dissolved in several solvents are depicted in figure.3c.These color changes indicate the presence of interactions between the CNPs material and the solvents used in this study.The selection of different solvents was made for specific reasons.There is a noticeable shift in the emission peak of CNPs when subjected to varying treatments in various solvents.These shifts occur when CNPs are dissolved in different solvents such as water, formamide, DMSO, and DMF, shifting from a wavelength of 497 nm to 522 nm (Figure .3a) [15].The emission peak shift of CDs depends on the polarity index value of the solvent.Figure .3b illustrates that the higher the polarity index value, the more polar the solvent becomes.The peak shift indicates that the interaction between CNPs and the solvent can influence the optical properties of CNPs, which in turn causes a shift in the fluorescence wavelength towards a longer wavelength (redshift) [16].

Conclusion
Concisely, CNPs have been successfully synthesized from CA and urea, exhibiting solvatochromic effects when dissolved in various solvents based on the polarity index solvents.The results in a redshift emission shift of 25 nm.They possess multiple carbon-carbon bonds and functional groups.They exhibit two absorption peaks in different regions and have a spherical shape with an average diameter of 50 -60 nm.

Acknowledgments
This work is fully supported by Riset PPMI KK, FMIPA Institut Teknologi Bandung, Fiscal Year 2023, Skema Postdoctoral KK Fisika Material Elektronik.R.U acknowledges Indonesian Endowment Fund for Education (LPDP), Ministry of Finance, Indonesia for doctoral scholarship program.

Figure 1 .
Figure 1.Schematic diagram of synthesis carbon nanoparticles (CNPs) under microwave radiation using solvent variation (water, DMSO, DMF, and formamide).Photographs of CNPs in solvent variation under daylight (left) and 365 nm UV (right) irradiation.

10thFigure 3 .
Figure 3. (a) PL spectra of the CNPs samples in various solvents, (b) The curve of solvent polarity index versus emission wavelength, (c) The sample of CNPs in various solvents without UV excitation and under UV excitation.