Fluorescent Nano-Biomass Dots: Ultrasonic-Assisted Extraction and Their Application as Nanoprobe for Fe3+ detection
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Biomass as sustainable and renewable resource has been one of the important energy sources for human life. Herein, luminescent nano-biomass dots (NBDs) have been extracted from soybean through ultrasonic method, which endows biomass with fluorescence property. The as-prepared NBDs are amorphous in structure with an average diameter of 2.4 nm and show bright blue fluorescence with a quantum yield of 16.7%. Benefiting from the edible raw materials and heating-free synthesis process, the cytotoxicity test shows that the cell viability still keeps 100% even if the concentration of the NBDs reaches 800 μg/ml, indicating the good biocompatibility of the NBDs. In addition, the fluorescence of the NBDs is very sensitive to Fe3+, which can be used for Fe3+ detection in terms of their health superiority. The limit of detection (LOD) of the proposed sensor was determined as 2.9 μM, which is lower than the maximum allowable level of Fe3+ (5.37 μM) in drinking water.
KeywordsNano-biomass dots Fluorescence Ultrasonic methods Nanoprobe Fe3+ detection
High-angle annular dark-field scanning transmission electron microscopy
Limit of detection
Nuclear magnetic resonance
Transmission electron microscopy
Ultrasonic extraction strategy
United States Environmental Protection Agency
X-ray photoelectron spectroscopy
Luminescent nanomaterials have achieved a wide variety of applications due to their unique optical properties, especially in light-emitting diodes, detectors, bioimaging, and metal ion detection [1, 2, 3, 4, 5, 6]. Various luminescent nanomaterials have been reported until now, such as semiconductor quantum dots (QDs), carbon nano-dots, and sulfur QDs, which have led to lots of advances in many fields [7, 8, 9, 10, 11, 12]. QDs as an excellent representative of luminescent nanomaterials have been used in many fields due to their excellent optical and electrical properties. Despite all this, the toxicity of QDs still limits their applications greatly [13, 14]. It is always of great importance to find greener and more sustainable nanomaterials with luminescence. Biomass is an original organic matter which can be produced via photosynthesis, and it is highlighted for its sustainable and renewable properties. Specifically, biomass is defined as the biodegradable fraction of products, waste, and residues of an organism [15, 16]. In the context of nanotechnology, biomass is usually used as a precursor, and it can be turned into nano-dots with some certain optical properties after special treatment. Compared with chemical precursors, the main components of biomass, especially edible biomass, are sugars and proteins, which are harmless in subsequent treatments. Therefore, the nano-biomass dots (NBDs) should be of high biocompatibility, which ensures their applications in biological and environmental fields without producing harmful substances.
Up to now, only biomass-derived fluorescent carbon nano-dots have been reported. Basically, some natural biomass such as leaves, egg white, and lemon juice were treated by hydrothermal method to synthesize fluorescence carbon nanoparticles [17, 18, 19]. There is also another kind of carbon nano-dots that exist in edible foods, which are produced in the further processing of natural biomass [20, 21]. Without exception, all of them involved typical processes of high-temperature carbonization. This process may involve a long time and high temperature, and it is difficult to achieve large-scale batch production . Compared with high temperatures, room temperature or low-temperature conditions are easy to be performed and maintain the original properties of biomass itself.
Nanoprobe is one of the important applications of luminescent nanomaterials . In view of the bright fluorescence and high biocompatibility, NBDs may be used as a kind of nanoprobe in the field of biology and environment. Fe3+ is an important metal ion in the human body for which they play a significant role in the synthesis of hemoglobin and myoglobin . But excessive Fe3+ accumulation in the body can lead to tissue damage and organ failure. Development of effective and greener sensing systems for qualitative and quantitative determination of Fe3+ is of great significance for clinical, medical, and environmental concerns. This allows us to consider whether biomass can be tailored into nano-dots with desirable properties directly from natural edible biomass without any processing. However, none of such luminescent NBDs have been reported to the best of our knowledge. Therefore, looking for more natural biomass precursors to obtain NBDs with desirable properties and high biocompatibility may take a step towards greener luminescent nanomaterials and Fe3+ detection.
Herein, luminescent nano-biomass dots (NBDs) have been demonstrated via ultrasonic extraction strategy (UES) from soybeans for the first time. The photoluminescence (PL) quantum yield (QY) of the as-prepared NBDs can reach 16.7%, and the NBDs show bright emission in the solid state. The cytotoxicity test shows that the NBDs have high biocompatibility. Additionally, the NBDs have been employed for Fe3+ detection for its fluorescence intensity dependence linearly on the Fe3+ concentration, and the limit of detection (LOD) can reach 2.9 μM.
Varieties of northeast soybeans in line with National Standard of the People’s Republic of China (GB1352-2009) were purchased from the local supermarket and washed several times with distilled water before use. Calcium chloride (CaCl2), manganese chloride (MnCl2), cupric chloride (CuCl2), cobaltous chloride (CoCl2), lead nitrate (Pb (NO3)2), and chromium nitrate (Cr(NO3)3) were purchased from Aladdin Ltd. (Shanghai, China). Ferric chloride (FeCl3), ferrous chloride (FeCl2), cadmium chloride (CdCl2), mercury dichloride (HgCl2), sodium chloride (NaCl), and zinc chloride (ZnCl2) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals are of analytical reagent (purity > 99.0%) and used as received without further purification.
Synthesis of NBDs
Firstly, 100 pieces of soybeans were washed with a mixture of alcohol and distilled water for 3 times to remove the impurity. Then the soybeans were placed into a beaker with 50 ml distilled water followed by ultrasonic for 2 h. During this process, the color of the solution changed from transparent to dark yellow, indicating the peel of soybean was tailored into nano-size to form NBDs. Then, the dark yellow solution was transferred into centrifuge tubes and centrifuged at 7000 rpm for 3 mins twice to remove large-size particles, after that the supernatant was filtered through a 0.22-μM membrane to remove large or agglomerated particles further. Whereafter, the solution was placed in a refrigerator followed by frozen treatment at − 5 °C for 6 h. Then, it was transferred to a lyophilizer at − 50 °C for 12 h to obtain the powders. The frozen powders were dispersed into water to form NBDs for further application.
The X-ray diffraction (XRD) pattern of the NBDs was recorded using an X′ Pert Pro diffractometer, in which X-rays were generated by a Cu-Kα source. A JEM-2010 transmission electron microscope (TEM) was employed to characterize the size and crystallinity of the NBDs. The fluorescence spectra of the NBDs were obtained with an F-7000 fluorescence spectrophotometer. The UV-Vis absorption spectra of the NBDs were obtained using a UH4150 spectrophotometer. The fourier-transform infrared (FTIR) spectra of the samples were recorded by a Thermo Scientific Nicolet iS10 FTIR spectrometer. The X-ray photoelectron spectroscopy (XPS) spectra of the samples were collected by using a Thermo Fisher Scientific ESCALAB 250Xi spectrometer equipped with an Al-Kα X-ray radiation source.
Photoluminescence Quantum Yield Measurement
The PL QY was tested using an F-9000 spectrofluorometer with integrating sphere. First of all, the NBD aqueous solution was diluted to an absorption intensity of below 0.1. Then, this aqueous solution was added into a fluorescence cuvette, placed in the integrating sphere, and excited with 370-nm monochromatic light. The fluorescence spectra were collected in the ranges of 430–450 nm. Meanwhile, the same fluorescence spectra for pure water were also recorded under identical conditions. Finally, the PL QY was calculated using fluorescence software based on the PL spectra of both the sample and the water.
Cellular Toxicity Test
The cytotoxicity of NBDs is evaluated by MTT (3-(4,5)-dimethylthiahiazo(-z-y1)-3,5-di-phenytetra-zoliu-mromide) methods. Cells were cultured in normal RPMI-1640 with 10% fetal bovine serum in 5% CO2 and 95% air at 37 °C in a humidified incubator. For cell viability measurements, HeLa cells were placed in 96-well plates and then incubated for 72 h. After the incubation of Hela cells with various concentrations of NBDs and CDs for 72 h, the cell viability was recorded.
Detection of Fe3+
A 1 ml of solution with different concentrations of Fe3+ was added into 1 ml of NBDs with 3 g/l solution before the PL measurements. The solutions were mixed thoroughly and left to react for 1 min at room temperature, and then recorded the associated fluorescence spectra. The PL measurements were performed under excitation of 370 nm.
Results and Discussion
Morphology and Chemical Composition
A possible mechanism for the formation of NBDs from soybeans was proposed based on the above analysis. First, some large particles of biomass suspended in the solution are broken into nanometer size by the ultrasonic concussion. Changes in solution before and after ultrasonic extraction treatment are shown in Additional file 1: Figure S1. Then, the protein in the soybeans was hydrolyzed to small molecular peptide and amino acid in the above process, and a lot of small molecule peptide chains are attached to the nano-size biomass to form the highly surface functionalized biomass dots. The functional groups on the surface of the biomass dots are the major contributors to fluorescence. According to the mechanism, the mung bean was also used as precursors, and blue fluorescent NBDs were also obtained, as shown in Additional file 1: Figure S2.
Sensing Properties of the NBDs Toward Fe3+
In summary, luminescent NBDs have been prepared from soybean via a heating-free UES approach. The NBDs show bright blue fluorescence with PL QY of 16.7%, and benefiting from the edible biomass and heating-free synthesis process, the cell viability still keeps 100% even if the concentration of the NBDs reaches 800 μg/ml. In addition, the fluorescence of the NBDs exhibits specific sensitivity to Fe3+, and the LOD can reach 2.9 μM. The low toxicity and high detection limit indicate that the NBDs are expected to find potential applications in biological and environmental systems.
The authors appreciate the Testing and Analysis Center, Zhengzhou University.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 21601159, 61604132, 61505033, 11374296), the National Science Fund for Distinguished Young Scholars (Grant Nos. 61425021).
Availability of Data and Materials
The datasets used for analysis can be provided on a suitable request, by the corresponding author. Electronic Supplementary Information (ESI) available.
KKL conceived the idea and CXS supervised the project. WBZ and KKL designed and conducted the experiments. WBZ, KKL, CXS, SYS, and RZ discussed the experimental data. WBZ wrote the manuscript. All of the authors discussed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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