Bifunctional Fluorescent Carbon Nanodots From Galangal as Sensors for Metal Ions and as a Nutrient Source for Peanut Growth

Summary

Bifunctional fluorescent carbon nanodots (C-dots) derived from galangal are demonstrated as sensors for metal ion detection and as a nutrient source for peanut plant growth. These findings highlight their potential as selective sensors for Fe^3⁺ and Mg^2⁺ and as nutrient supplements to enhance peanut growth.

Abstract

This study demonstrates the bifunctionality of fluorescent carbon nanodots (C-dots) derived from galangal as both sensors for metal ions and a nutrient source for peanut growth. The C-dots were fabricated via a carbonization method at 200 °C for 30 min and exhibited blue fluorescence in ethanol and aqueous solutions. Characterization using absorption, photoluminescence (PL), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD) confirmed successful formation. The C-dots showed absorption peaks at 275 nm (ethanol) and 270 nm (aqueous), with an average diameter of 5.74 nm and a standard deviation of 1.11 nm. PL intensity was significantly quenched upon interaction with Fe^3⁺ and Mg^2⁺ ions, enabling their detection with limits of detection of 2.98 µM and 3.3 µM, respectively. The application of C-dots enhanced peanut growth, as evidenced by increased chlorophyll content and biomass compared to untreated plants. These results indicate that galangal-derived C-dots function as effective nutrient sources for vegetative growth. Overall, this approach provides an economical and environmentally friendly strategy for developing bifunctional C-dots for metal ion sensing and agricultural applications.

Introduction

Water is an essential resource for all living organisms, including humans, as it contains essential minerals required for maintaining metabolic processes. The concentration of metal ions such as ferric (Fe^3⁺), magnesium (Mg^2⁺), zinc (Zn^2⁺), copper (Cu^2⁺), and mercury (Hg^2⁺) in water must be carefully regulated to ensure suitability for consumption. Deviations from optimal concentrations, whether deficient or excessive, are known to cause various diseases in humans and contribute to environmental pollution^1,2,3. In addition to water, plants serve as a primary food source for living organisms, particularly humans and animals, supporting essential metabolic processes. One important agricultural crop is the peanut plant (Arachis hypogaea L.), a leguminous species native to South America, including Brazil, Bolivia, Paraguay, and northern Argentina, and now cultivated worldwide, including in Indonesia. Over the past thousand years, peanut seeds, rich in protein, have become essential to food production and remain a critical agricultural commodity. However, increasing peanut production remains challenging due to environmental factors such as prolonged drought, land degradation, pest infestations, and diseases, including leaf spot and rust, as well as limited technological implementation^4,5.

To address the dual challenges of monitoring metal ion concentrations in water and enhancing peanut plant growth, carbon nanodots (C-dots) represent a promising solution. These materials can function as sensors for detecting trace concentrations of metal ions and as agents to enhance plant growth. Fluorescent C-dots, a class of carbon-based nanomaterials, have been widely applied in bioimaging⁶, sensing^2,7, drug delivery⁸, and as nutrient sources for plant growth^9,10. Their physicochemical properties, including high photostability, strong fluorescence, excellent biocompatibility, nanoscale size, resistance to photobleaching, and low toxicity, make them highly suitable for biological applications^11,12. Additionally, C-dots exhibit high water solubility, biodegradability, abundant precursor availability, and low production cost^9,11, further enhancing their applicability in sensing and agriculture.

The use of C-dots derived from organic material and their associated waste has been extensively investigated as sensors for the detection of metal ions and as a nutrient source to increase agricultural production. This increasing interest is primarily due to their excellent physicochemical properties, such as high photostability, strong fluorescence, excellent biocompatibility, nanoscale size, resistance to photobleaching, and low toxicity, which collectively make them highly suitable for various biological applications^11,12. Additionally, C-dots have high water solubility, excellent biodegradability, abundant precursor availability, and low production costs^9,11. The synergistic combination of low toxicity, biocompatibility, strong PL, and high photostability renders C-dots highly promising as an excellent fluorescent probe for cellular multi-color imaging¹¹ and as a nutrient source to enhance plant growth, thereby contributing to the increased agricultural productivity.

Numerous studies have explored the use of C-dots derived from organic materials and waste as sensors for metal ion detection and as nutrient sources for improving agricultural productivity. C-dots prepared from p-phenylenediamine using a hydrothermal method were successfully applied for Fe³⁺ detection with a limit of detection (LOD) of 0.85 µM¹³. Similarly, C-dots fabricated from amido black 10B were used to detect Fe³⁺ ions in lake water, achieving an LOD of 1.84 µM¹⁴. C-dots derived from dragon fruit peel have also been reported for Zn²⁺ detection with an LOD of 3.2 µM⁷. In agricultural applications, C-dots synthesized from glucose and urea increased rice yield by 14.8%⁹. Growth enhancement of Ipomoea aquatica using C-dots derived from dragon fruit peel and improved growth of hydroponically cultivated romaine lettuce using C-dots have also been reported¹⁰˒¹⁵. A previous study reported the fabrication of C-dots from galangal using a microwave method and investigated their optical properties as a function of processing time¹⁶; however, their application as sensors and nutrient sources was not explored.

In this study, C-dots were fabricated from galangal (Alpinia officinarum) using a carbonization method^7,12. The optical and physicochemical properties of the prepared C-dots, including photostability and zeta potential, were analyzed, along with their applications as sensors for metal ion detection and as nutrient sources for evaluating vegetative growth in peanut plants. Galangal is a perennial plant widely cultivated in Asia, particularly in Indonesia and Thailand. It is known for its applications in food and medicine due to its carminative, stomachic, antispasmodic, and antimicrobial properties¹⁶. Phytochemical analysis indicates that galangal contains terpenes and phenolic compounds rich in carbon, nitrogen, and oxygen, making it a suitable precursor for C-dot synthesis^16,17. Thus, galangal represents an abundant, inexpensive, and renewable carbon source. The objectives of this study are to analyze the optical and physicochemical properties of galangal-derived C-dots and to evaluate their applicability as sensors for metal ion detection and as nutrient sources for enhancing peanut plant growth. These materials exhibit dual functionality, a property referred to as bifunctional fluorescent C-dots. The testable hypothesis is that peanut plants treated with C-dots exhibit improved growth compared to untreated plants.

Carbonization of galangal at 200 °C for 30 min produced fluorescent C-dots that were readily dispersed in ethanol and aqueous solutions, and emitted blue fluorescence under UV illumination at 365 nm. Characterization using absorption (Abs), photoluminescence (PL), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD) confirmed successful synthesis and favorable optical and structural properties. The PL intensity decreased with increasing concentrations of Fe³⁺ and Mg²⁺ ions, demonstrating sensitivity and selectivity, with LOD values of 2.98 µM and 3.3 µM, respectively. Furthermore, application of galangal-derived C-dots significantly enhanced vegetative growth of peanut plants, as indicated by increased chlorophyll content and biomass compared to untreated controls.

Protocol

This study did not involve human participants or animal subjects. All plant experiments were performed in accordance with applicable institutional and national guidelines for plant research and biosafety. The chemicals and equipment are listed in the Table of Materials.

1. Fabrication of C-dots from Galangal

The fabrication of carbon nanodots (C-dots) from galangal was performed using a carbonization method^7,18 with minor modifications. Galangal powder (20 g) was carbonized at 200 °C for 30 min to produce a dark sample. After cooling to room temperature, the sample was crushed to obtain a fine powder. A portion of the dark galangal powder (0.5 g) was added to 5 mL of ethanol, and the mixture was sonicated for 1 h to obtain a homogeneous dark solution. Subsequently, 9 mL of ethanol was added, and the mixture was centrifuged at 805 × g for 30 min at room temperature to remove larger particles. After centrifugation, the sample was filtered through a 0.22 µm Whatman filter to remove residual large particles, yielding a clear supernatant containing fluorescent C-dots in ethanol solution.

The preparation of C-dots in aqueous solution followed the same procedure as for the ethanol solution. However, the yield of the dissolved galangal powder in aqueous solution was lower than that obtained in ethanol solution. Under illumination with a 365 nm UV lamp, the as-prepared C-dots in both ethanol and aqueous solutions exhibited blue fluorescence. The optical and physicochemical properties of the prepared C-dots, including absorption (Abs), photoluminescence (PL), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), morphology, size distribution, and zeta potential, were subsequently characterized. Finally, the prepared C-dots were evaluated for their applicability as sensors for metal ion detection and as nutrient sources for promoting peanut plant growth.

2. Synthesis of C-dots from galangal with metal ions

The interaction of C-dots with ferric (Fe^3⁺) ions was investigated. Specifically, 1 mL of C-dot dispersion in ethanol was mixed with 3 mL of ethanol and 0.75 mL of phosphate-buffered saline (PBS, 1 M, pH 7.0), followed by the addition of 100 µL of Fe^3⁺ solution at varying concentrations (0–10 µM). Photoluminescence (PL) spectra were recorded using an excitation wavelength of 350 nm. Fourier transform infrared (FTIR) spectra were also measured to evaluate potential interactions between C-dots and Fe^3⁺ ions. All experiments were conducted at room temperature.

The same experimental procedure was applied to investigate interactions between C-dots and other metal ions, including magnesium (Mg^2⁺), zinc (Zn^2⁺), and copper (Cu^2⁺).

3. The use of the as-produced C-dots for the growth of the peanut plant

Black soil (4 kg) and rice husks (1 kg) were mixed and placed in polybag containers. The water-holding capacity of the soil mixture was measured to determine the volume of water it could absorb. Peanut seeds were obtained from a local market in Kupang, East Nusa Tenggara, Indonesia; the cultivar was not specified because the supplier did not provide labeling. Seeds had a reddish-brown to off-white seed coat, measuring 6–7 mm in length and 4–5 mm in width. Germination was assessed on moist filter paper in Petri dishes at 25–28 °C for 5–7 days, with germination defined as radicle emergence ≥2 mm, yielding a germination rate of 99%. The seeds were soaked in water for 12 h and then planted at a depth of 3 cm in each polybag. Each polybag received 1200 mL of water daily. C-dots were applied weekly at volumes ranging from 1 to 10 mL for two months. Each treatment volume included two replicates. A total of 30 polybags were used: 10 without C-dots (control) and 20 with C-dots. Untreated seeds and seeds treated with C-dots were cultivated under identical natural environmental conditions in a greenhouse. Environmental parameters, including temperature (25-30^oC), relative humidity, light intensity, and photoperiod, were monitored and controlled naturally throughout the experiment to ensure consistency and reproducibility.

Plant height was measured weekly, while biomass and chlorophyll content were measured after two months. Data were analyzed using analysis of variance (ANOVA) at p < 0.05, and mean separation was performed using Duncan's Multiple Range Test (DMRT) at the same significance level.

Results

Preparation and general characteristics of C-dots from Galangal

Figure 1 illustrates a schematic of the preparation of highly fluorescent carbon nanodots (C-dots) from galangal in ethanol and aqueous solutions and their application as sensors and nutrient sources for peanut plant growth. The C-dots were prepared from galangal using a carbonization method. Galangal was carbonized at 200 °C for 30 min to obtain a dark sample, indica...

Discussion

The present study demonstrates the successful fabrication of fluorescent carbon nanodots (C-dots) from galangal using a simple carbonization method. The prepared C-dots exhibited desirable optical, structural, and biological properties, supporting their bifunctional application as sensors for Fe³⁺ and Mg²⁺ ions and as nutrient sources for peanut plant growth. The enhanced vegetative growth observed in peanut plants treated with C-dots supports the proposed hypothesis that C-dots improve peanut plant...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Aquadest	Water One	A-1708-5LT	Solvent for preparation of aqueous C-dot dispersions
Black soil and rice husk	Local product from Kupang City, East Nusa Tenggara Province, Indonesia	N/A	Planting medium for peanut cultivation
Centrifuge	LC-04S	N/A	Used to separate C-dots from carbon residues
Copper chloride dihydrate	Emsure	CAS No. 10125-13-0	Metal ion source for PL quenching experiments
Dynamic light scattering instrument	BeNano 180 Zeta Pro	N/A	Used for zeta potential measurements of C-dots
Ethanol absolute for analysis	Emsure	CAS No. 64-17-5	Solvent for C-dot dispersion and synthesis
Ferric chloride dihydrate	Emsure	CAS No. 10025-77-1	Fe3+ ion source for sensing experiments
FT/IR-4200 Fourier-transform infrared spectrophotometer	JASCO	FT/IR-4200	Used to measure FTIR spectra of C-dots
Galangal (Alpinia galanga)	Local product from East Nusa Tenggara Province, Indonesia	N/A	Carbon precursor for preparation of fluorescent C-dots
Magnesium chloride hexahydrate	Emsure	CAS No. 7791-18-6	Mg2+ ion source for sensing experiments
Phosphate-buffered saline (PBS)	Buana Laboratories	N/A	Buffer solution used to maintain pH
Polybag	Local product from Kupang City, East Nusa Tenggara Province, Indonesia	N/A	Container used for peanut cultivation
Spectrofluorometer	SHIMADZU	RF-6000	Used to measure photoluminescence spectra of C-dots
Transmission electron microscope (TEM)	JEOL	JEM-2100Plus	Used to investigate morphology and particle size of C-dots
Ultrasonic cleaner	Jewelry Cleaner	N/A	Used for homogenization and dispersion of C-dots
UV lamp (365 nm)	Sofim	N/A	Used for fluorescence visualization of C-dots
UV spectrophotometer	JASCO	UV-570	Used to measure absorption spectra of C-dots
X-ray diffractometer (XRD)	Bruker AXS	N/A	Used to measure XRD patterns of C-dots
Zinc chloride	Emsure	CAS No. 7646-85-7	Zn2+ ion source for PL quenching experiments