Fluorescent carbon quantum dots (CQDs) are a new class of carbon-based nanomaterials, with a spatial size generally less than 10 nm. They have attracted great attention in many research directions due to their various properties [1, 2]. Carbon quantum dots had environmental friendliness, broad absorption spectrum, tunable optical properties, good charge transfer/separation ability, high fluorescence and good quantum yield as well as various biological properties [3, 4]. Therefore, they appear as promising materials for a wide range of applications such as optoelectronic applications [5], fluorescent probes, pH sensor [6], cell imaging [7], photocatalysis [8], optical thermometry [9]. Nowadays, several methods have been developed to prepare fluorescent carbon quantum dots, such as solvothermal/hydrothermal treatment [10], arc discharge [11], laser ablation [2, 12], electrochemical [10], ultrasonic [13], and microwave [14]. Among these methods, hydrothermal processing is one of the best and low-cost green chemistry methods to prepare large-scale fluorescent CQDs using biomass resources as carbon sources [15]. In addition to, this method is easy to set up and produces uniformly shaped particles with high yield [16]. Therefore, efforts have been inspired to synthesize fluorescent CQDs using different natural resources such as castor seeds [9]. The castor seed plant (scientific name: Ricinus communis) was a popular ornamental plant in gardens due to its ability to produce large quantities of fertile seeds, which are dispersed effectively. Consequently, this plant has become an important environmental weed that is found in many diverse locations. The castor seeds contain the oil as well as the toxic protein ricin [17].
To our knowledge, the hydrothermal approach has not been previously used to synthesize N-CQDs from castor seeds. Moreover, castor seed-based CQDs have never been applied in pH sensing or cell imaging. The importance of pH is well known in a variety of fields, including biology, agriculture, industry, and the environment. In addition to everyday applications such as food and beverage analysis and disease detection, pH monitoring is essential for scientific research. To bypass the drawbacks of typical organic fluorescent dye, such as background autofluorescence and photobleaching, pH-sensitive nanoprobes based on organic dyes and nanocarriers have been created. However, since organic dyes and nanocarriers have large particle sizes and low biocompatibility, it is still difficult to provide long-term pH monitoring without biological damage [18]. Carbon quantum dots with small size, excellent photostability and good biocompatibility have the potential to provide solutions for pH sensing in many fields especially pharmaceutical and in vivo medical applications [6].
Bioimaging refers to a non-invasive technique to visualize a biological system in real time. It includes the study of subcellular arrangement and whole cells as well as tissues and multicellular organisms. CQDs have been identified as attractive candidates for cell imaging due to their greater tunable photoluminescence (PL), lower toxicity, hydrophilic feature, and great photochemical stability under UV illumination. Most CQDs effectively generate blue emission with short excitation spectra, which severely limits bioimaging. Fortunately, several studies have shown that heteroatom doped such as N-CQDs have extended wavelengths and multicolor fluorescent, making them suitable for real-time and longer-time cell imaging [16, 19].
The main objective of the present work is to synthesize the N-CQDs using a hydrothermal method and castor seeds as the sole carbon source. One goal is to characterize CQDs prepared using different experimental techniques and systematically examine the fluorescence of the N-CQDs for their potential for applications in pH sensing and cell imaging. As one of the objectives, cytotoxicity and antimicrobial activity measurements of the synthesized N-CQDs will be performed to enhance the advantages of their applications.