废米糠制备炭纳米颗粒及其对金属离子的光学传感性能

Peggy Zhen Zhen Ngu,　Stephanie Pei Phing Chia,Jessica Fung Yee Fong,2,　Sing Muk Ng,2

(1.Faculty of Engineering, Computing, and Science, Swinburne University of Technology Sarawak Campus,Jalan Simpang Tiga93350, Kuching, Sarawak, Malaysia;2.Swinburne Sarawak Research Centre for Sustainable Technologies, Swinburne University of Technology Sarawak Campus,Jalan Simpang Tiga93350, Kuching, Sarawak, Malaysia)



废米糠制备炭纳米颗粒及其对金属离子的光学传感性能

Peggy Zhen Zhen Ngu1,Stephanie Pei Phing Chia1,Jessica Fung Yee Fong1,2,Sing Muk Ng1,2

(1.FacultyofEngineering,Computing,andScience,SwinburneUniversityofTechnologySarawakCampus,JalanSimpangTiga93350,Kuching,Sarawak,Malaysia;2.SwinburneSarawakResearchCentreforSustainableTechnologies,SwinburneUniversityofTechnologySarawakCampus,JalanSimpangTiga93350,Kuching,Sarawak,Malaysia)

摘要：以废米糠为原料，浓硫酸为脱水剂，通过炭化法制备出炭纳米颗粒(CNPs)，探讨CNPs荧光发射特征、金属离子的淬灭效应以及作为Sn(II)离子传感材料应用。CNPs产率最佳条件为：硫酸浓度12 mol/L、加热温度120 ℃及恒温时间30 min。样品在水中强蓝光的最大发射波为439 nm。通过加入金属离子，使金属离子与CNPs表面间形成复合物而淬灭荧光。Sn(II)离子对CNPs荧光具有显著的淬灭效应。Sn(II)离子浓度对淬灭效应符合Stern-Volmer线性关系，Sn(II)离子为6.13 mmol/L。Sn(II)离子的检测限为18.7 μmol/L。

关键词：炭纳米颗粒； 荧光； 淬灭； 传感； 金属离子

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).

1Introduction

Nanomaterials are of great scientific interest owing to their small dimension and unique physical properties that are different from the bulk materials. Bulk materials often show constant physical properties while nanomaterials have the size-dependent properties such as the quantum confinement effect that gives rise to the fluorescence property for some semiconductor nanoparticles[1]. The colour of the emission can be tuned by changing the average size of the nanoparticles[2]. Besides, the large surface area due to the small size has enhanced affinity toward the solvent molecules. This phenomenon allows the formation of a colloidal solution since the nanomaterials can be dispersed homogenously within the solvent and interface interactions between the nanomaterials and others existing species within the solvent occur under this condition. Hence, with a synthesized of these unique physical and colloidal properties, the nanomaterials can be used for several applications such as sensing[3], drug delivery[4]and environmental remediation[5, 6].

Various nanomaterials have been synthesized and the effort to produce new novel nanomaterials is still on-going. The main motivation is to incorporate better physical properties and chemical functionalities into the nanomaterials to improve its performance in applications. In addition, there is also a need to search for new nanomaterials that are more sustainable, which not only non-toxic, biocompatible and environmental friendly, but also the synthesis adopting green production methods and using renewable precursors. Carbon nanoparticles (CNPs) are the latest alternative of nanomaterials that have been discovered and portrayed several advantages over other existing nanoparticles. Similar to the well-known quantum dots, CNPs show bright fluorescence, high photostability, and tuneable excitation and emission spectra[7, 8]. In addition, CNPs are less-toxic, soluble in water and have good biocompatibility, non-blinking fluorescence, chemical inertness[9]. Their low molecular weights as well as small sizes make them a good candidate for drugs delivery[10, 11]. The work by Baker[12]has reported on the potential of CNPs to be used for bio-imaging, drug delivery and diagnostic tools. In a separate study, Yang et al.[13]has demonstrated that CNPs showed no or low toxicity when testedinvivoon mice, hence had less concern on the safety issue even for some applications within the human body.

In the early stage of CNP development, the synthesis methodologies taken are based on the top-down approach. It requires the formation of CNPs from bulk carbon sources such as bulk graphite via various harsh mechanical processes. One of the examples is through the use of laser ablation to produce the nano-sized carbon particles, where strong laser beam is focused onto a carbon target, eventually causing the formation of arc-discharge soots. The soots upon surface passivation using oxidizing agents often showed good fluorescence and water soluble properties. In fact, this method has been employed in the work by Xu et al.[14], which is believed to be the first work reported on the isolation of fluorescent CNPs. Although the top-down approach produces purer CNPs, the instrumental setup to perform the mechanical cracking of carbon is often sophisticated and expensive. Small scale laboratories without high technological facilities installed and sufficient financial funding supports will not be able to produce CNPs through this route. Therefore, continuous effort in search for alternatives has led to the more sustainable bottom-up approach, where CNPs are formed from simple molecular precursors via chemical process. This approach utilizes precursors that are often readily available and cheap in cost. Besides, the methods adopted for the bottom-up synthesis are also much easier with fewer steps, more basic laboratory setups and lower energy consumption than the top-down ones. Liu et al.[15]reported a facile synthesis of CNPs using candle soots, while Sahu et al.[16]demonstrated the production of CNPs from the carbonization of orange juice.

Herein, we propose and demonstrate a simple yet novel method to synthesis CNPs from waste rice husks based on a thermal-assisted acid carbonization approach using sulfuric acid. The proposed method focuses on the aspect of sustainability by adopting green chemistry for synthesis, using safe and renewable resource as starting precursor. Rice husk is a good candidate as starting precursor since it is considered as agricultural waste, renewable resource, considerably cheap, and can be obtained easily in bulk. Majority of rice-producing countries burn the rice husks in open piles that can cause serious air pollution. Others dump them at open landfills, where these rice husks are left to rot and eventually can lead to the production of methane, a greenhouse gas that causes global warming[17].

The colloidal interactions of the CNPs with some metal ions have been investigated with the attempt to utilize such interfacial phenomena for a real application. The synthesis method suggested in this study is novel and sustainable, but the effort will be underutilized if the CNPs have no further usage. Metal ions were selected in the study owing to their positively charged nature that poses a high probability to interact with the surface of the CNPs, as the surface is rich with carboxyl groups after the acid carbonization process. The effect of the intersurfacial interactions of CNPs with metal ions on the fluorescence emission was monitored and characterized towards optical sensing application. Particularly in this study, the fluorescence of the CNPs was characterized towards the detection of stannous ions (Sn(II) ions).This work suggests an option to convert cheap agricultural waste into advanced optical sensing nanomaterial of high commodity value. In addition, the CNPs can act as an alternative to replace the use of some existing fluorescent dyes or quantum dots that are produced from far less sustainable approaches as compared with the CNPs reported in this work.

2Experimental

2.1Chemicals and reagents

All chemicals involved in this study were of analytical grade and used as received without further purification unless otherwise stated. Ultrapure water (～18.2 MΩ, 25 ℃) was obtained from a Milipore Mili Q-system and used as solvent throughout the study. Rice grains were obtained from a local rice-hulling mill (Sibu, Sarawak) and the husk was carefully peeled off from the rice grain. The rice husks were washed for 3 times and soaked in water for at least 12 h, before rinsed with water and dried in oven for 30 min before use. Concentrated sulfuric acid (H2SO4, with a concentration of approximately 18 mol/L) was used as dehydrating agent for the carbonization. For the intersurfacial interaction study with metal ions, heavy metal ions stock solutions were prepared in deionized water from the respective salts of (Cu(NO3)2, SnCl2, Ni(NO3)2, Al(NO3)3,Co(NO3)2, Pb(NO3)2, AgNO3and HgCl2) that were all purchased from R & M Marketing, Malaysia.

2.2Instrumentation

Fluorescence intensity was recorded using a standard lab based spectrofluorometer (CARY Eclipse, Varian) set under the fluorescence mode. To do the measurement, the sample was transferred in a quartz cuvette with a path length of 10 mm and four-side windows cleared and polished . The cuvette was placed in the spectrofluorometer and the emission was recorded with the slits set at 5.0 and 10.0 nm for the excitation and emission paths, respectively. The pH was adjusted using acid and base and the value was monitored with a pH meter (Mettler Toledo SevenEasy). Carbonization temperature was controlled using a standard laboratory furnace (Carbolite ELF 11/14B). The separation for nanoparticles was performed by centrifugation method using a centrifuge system (Eppendorf Minispin).

2.3Synthesis of CNPs

The bottom-up approach was employed to synthesize the CNPs via simple carbonization of rice husks using H2SO4. In this study, high temperatures were adopted to assist and speed-up the carbonization. In brief, 0.200 g of cleaned and dried rice husks were transferred into a small beaker, to which 2.0 mL of H2SO4(12 mol/L) was added. The beaker was then wrapped using an aluminium foil and heated in an oven at 120 ℃ for 30 min under air, which resulted in the formation of black residue at the end of the heating. The residue was divided approximately into two portions and each transferred into a 2.0 mL microcentrifuge tube. The beaker was rinsed using 2.0 mL of ultrapure water and likewise divided into two portions and transferred into the same two tubes containing the initial residue. The tubes were centrifuged at 13 400 r/min for 15 min. The supernatant of yellowish-brown in colour was then collected using Millipore syringe filters (size of 0.1 μmol/L) and redispersed into ultrapure water. Strong blue luminescence was observed upon irradiation of the redispersed solution under an UV transilluminator. The sample solution was kept in an opaque bottle and stored in dark and cool place for further study.

2.4Optimization of synthesis variables and colloidal interaction study

Important experimental parameters governing the synthesis of the CNPs were investigated. The concentration of H2SO4used to carbonize the rice husks was varied from 3.0 to 18.0 mol/L, while other parameters remained constant. The fluorescence of the final isolated and collected sample from each batch was compared in order to identify the best concentration of acid used to produce the CNPs. Similarly, the same protocol was adopted for the optimization of temperature and time used for the carbonization with the temperature varied from 25 to 120 ℃ and the standing time varied from 15 to 75 min.

The effect of metal ions towards the fluorescence of the CNPs was investigated by recording the change in the intensity before and after the addition of a fixed amount of the metal ions into the solution. The metal ions chosen for the study were Cu(II), Ni(II), Al(III), Co(II), Pb(II), Ag(I), Sn(II), and Hg(II). The metal ions that showed the most significant quenching was further characterized to generate the relevant analytical information. This was performed by preparing a series of solutions with a fixed amount of CNPs that were then added with an increasing amount of the metal ions. On the detailed procedure, 60 mL of as-prepared CNP solution was used and mixed with the prefixed volume of the stock solution (0.1 mol/L). The mixture was made up to a final volume of 3.0 mL, stirred and transferred to a cuvette for fluorescence measurement. In order to achieve a better sensitivity and a wider dynamic range, the initial fluorescence intensity of the CNPs was optimized to be the strongest within the scale window of the instrument. This was done by varying the amount of CNPs used and the pH value of the mixture.

3Results and discussion

3.1Characterization of CNPs

In this study, the CNPs were produced via dehydration of rice husks by concentrated H2SO4. Once added with H2SO4, the rice husks changed into black residue and majority dissolved slowly into the acid media after a gentle swirl of the beaker, forming a thick black solution. H2SO4is a powerful dehydration agent and can remove water moiety from organic compounds. The carbonization often produces water molecules, carbon residues and volatile gases. In this study, the product isolated after the extraction was mostly the CNPs as the mixture was found to be glowing under an UV lamp (inset, Fig. 1). Further study performed using a spectrofluorometer has confirmed the fluorescence band with a peak at 439 nm when the product was excited at an optimum wavelength of 358 nm (Fig. 1). At the same excitation wavelength, H2SO4, ultrapure water and solution pre-soaked with rice husks showed low or no fluorescence. This has supported that the fluorescence recorded from the sample isolated was due to the carbonaceous product dispersed as a colloidal solution, while not from the starting materials or reagents used for the synthesis. Although there was no report yet of the fluorescent product formed from the dehydration of rice husks using the method suggested here, the observation of fluorescence from CNPs has been well reported[18, 19]. This confirmed that the product isolated was colloidal solution containing CNPs.

The origin of the fluorescence for CNPs remains unclear till date. Nonetheless, some studies have suggested that the origin was due to the surface defect states on the CNPs[20]. This could be the possible cause of the fluorescence observed from the CNPs isolated in this work. The CNPs in this study showed strong fluorescence even without surface passivation process that is usually reported to be a crucial step to generate fluorescence from CNPs[21]. This could be due to the carbonization using strong acid that has simultaneously oxidized the surface of the CNPs, introducing lattice defects onto the surface with different oxidation states. These defects were significant to create band gaps due to the small size and high surface area of the CNPs, which allowed electronic transitions to occur and subsequently generated the fluorescence. This was further supported by the observation that the emission peak was not shifted, but only the change in intensity as the excitation energy provided was varied from 200 to 400 nm.

Fig. 1　Spectra of the (a) CNPs emission monitored at 439 nm

3.2Optimization of synthesis parameters

The carbonization of the rice husks was optimized to increase the yield of the CNPs. Several key parameters governing the synthesis procedure were investigated, which included the synthesis temperature, heating time, concentration of acid, and the cycle of extraction. Since the CNPs were the only species that showed fluorescence, the intensity was used directly as the measureable variable for the optimization studies. The increment of intensity was caused by the increasing amount of CNPs and vice versa.

3.2.1Effects of temperature

In this study, the carbonization was performed via thermal assisted hydrolysis using strong acid. The reaction chamber was set accordingly with different temperatures during the synthesis. In general, it was found that the intensity of the isolated sample from each batch increased with temperature during the carbonization (Fig. 2). This directly could be correlated to the amount of CNPs in solution that was isolated from high carbonization temperatures. Usually, a simple pyrolysis conversion of biomass containing hemicellulose and cellulose to carbon is reported to occur only at around 150 ℃ and above, while lower temperatures usually show less conversion[22]. Sufficient thermal energy is required for the depolymerization of the biomass network. However in this study, the carbonization was effective even at temperatures lower than 150 ℃ and this could be due to the presence of the strong acid that has assisted the depolymerization. The acid could breakdown the initial three-dimensional (3D) structure of the polymer in the biomass into small fragments, thus lowering down the activation energy for the pyrolysis[23]. Smaller clusters promote faster thermal degradation, more homogenous carbonization and higher conversion rate due to larger surface area. In this study, carbonization temperature of 120 ℃ was chosen for the synthesis since the yield of CNPs is high at this temperature.

Fig. 2　The effect of temperature during carbonization on the

3.2.2Kinetic of carbonization

The kinetic for the carbonization was studied by monitoring the fluorescence intensity of the colloidal solutions that were isolated from samples carbonized for different times. The yield showed an increment for the first 30 min, but suffered a slight reduction of around 20% and stabilized after that with the carbonization time (Fig. 3). As the ingredients were first mixed, the carbonization will proceed at a steady rate to convert the rice husks into CNPs. However as the process was continued longer at the same condition, some of the small or fragile fragments of CNPs could be converted further into ashy products. Ashes are basically mineral oxides that show no fluorescence. Besides, the prolonged carbonization could also cause over-heating that can destroy the surface defects responsible for the fluorescence. This phenomenon was observed by Zhai and co-workers in their work of synthesizing carbon dots using microwave assisted pyrolysis[24]. Based on the results obtained from this study, the optimum carbonization time of the waste rice husks was fixed at 30 min.

In this study, H2SO4was used to carbonize the waste rice husks as it has strong dehydration capability and oxidation power for the surface of the CNPs to give rise to the fluorescence. The state of the art is to obtain an apppropriate degree of carbonization of the rice husks since both under and over carbonizations could lead to the loss of fluorescence. The degree of carbonization was controlled by varying the hydrolysis strength of the acid by diluting.

Fig. 3　The kinetic profile of the carbonization using

Fig. 4 shows the fluorescence intensity of the samples that were prepared from different concentrations of H2SO4. Obviously, the acid concentrations of 3 mol/L or lower failed to convert the rice husks into CNPs due to the insufficient dehydrating strength. At this concentration, naked eye observation failed to detect any significant change in appearance of the mixture over a period of 2 h. However, as the concentration was increased gradually from 3 to 12 mol/L, a significant increment in the fluorescence intensity was observed, indicating the increase of CNP yield. Similar trend was also recorded in the work by Wang et al[25]. As the concentration of the H2SO4was gradually increased to 18 mol/L, a significant drop in intensity was observed instead of a continuous increment. This was due to the harsh condition caused by the concentrated acid that had converted the hemicellulose, cellulose and lignin in the rice husks into majority of ashes instead of CNPs. This was not the appropriate acid contentration as the main product aimed was the CNPs, while not the low fluorescence ashy products. Besides, in line with the green synthesis intention, the use of less amount of H2SO4is preferable. Thus this study utilized the H2SO4of 12 mol/L for all the subsequent syntheses.

3.2.4Isolation of CNPs

In this study, the CNPs were isolated from the carbonized residue via aqueous solvent extraction method. The residue was added with deionized water to disperse the CNPs into the aqueous media and the solution was then separated from the bulk residue using centrifugation. The sample was collected using a syringe filter. The solution isolated was clear and slightly yellowish in colour. Since only nano-sized particles can form colloidal solution, majority of the product extracted will consist of the CNPs with sizes in the nanometer range. The extraction step was repeated for several cycles in order to isolate most of the CNPs from the residue. Fig. 5 shows the progress of the extraction and it was clear that at least 5 cycles of extraction were required to isolate completely the CNPs from the residue. Further extraction was not required since the product collected from additional cycles showed sufficiently low fluorescence, indicating a negligible amount of the CNPs in the extract.

Fig. 4　The effect of the concentration of H2SO4 acid used for the

Fig. 5　Recovery of CNPs via repeated cycles of

3.3Condition of colloidal solution

3.3.1Effects of pH value

Since the surface of the CNPs produced by acid carbonization was rich in carboxyl and hydroxyl functional groups, the variation in the ionic condition of the solution can significantly alter the interface chemistry of the CNPs. This could subsequently cause some change in the physical properties of the CNPs. In view of this, the effect of pH value towards the fluorescence of the CNPs was investigated. The pH value of the solution containing CNPs was adjusted to either acidic or basic accordingly using either acid (H2SO4) or base (NaOH). In general, the fluorescence showed highest intensity at the neutral pH value of 6-8. Under proton-rich conditions (pH ≤ 5), the oxidized surface will be potentially protonated and subsequently the net negative charge on the surface will be reduced. Under such condition, electrostatic repulsion force between the nanoparticles will become weaker, thus promoting their aggregation due to van der Waals force and turning off the fluorescence. Besides, Liu et al.[15]has suggested that the decrease of fluorescence intensity at low pH condition could be due to the formation of intermolecular hydrogen bonds. These hydrogen-bonded aggregates were often less soluble in aqueous and could cause a decrease in the fluorescence intensity[26, 27].

第二，胡适认为传统“三不朽”中，真能立功立德立言终究只是少数人，所以只是“寡头之不朽”；而他主张“所有人”，包括“无量平常人”都能不朽。胡适提出“社会的不朽论”的直接契机是母亲的离世。他的母亲是一个极普通的女人，也是对其影响至深的人。平常人，尤其是女人，在过去的历史观中是被忽略或遗忘的。但现代是呼唤平等、呼唤“无量平常人”走上历史舞台的时代。每个作为个体存在的“小我”在其一生有限的时间中，都会留下自己独特的历史印记。胡适的“不朽”摈弃了贵贱有别的生命价值和帝王将相的英雄史观，把普通人纳入历史主体的范畴，这无疑展现了平等的时代精神。对他个人而言，母亲是他一生最难忘最温暖的怀念。

At high pH condition, the fluorescence of CNPs could be quenched by the hydroxide ions due to the formation of less soluble hydrated products[28]. Such phenomenon was usually observed in the quantum dot system, where high pH caused the formation of precipitate, leading to the loss of fluorescence[29]. Besides, the surface de-protonation could create excessive negative charges on the surface that favor the formation of negatively charged double layers[30, 31]. The double layers could induce and promote photo-induced electron transfer, where the excess electrons from the charged surface could fill up the holes created after electrons were excited to higher energy levels. Thus, this could retard the relaxation transition that caused the fluorescence. Instead, the transitions can undergo non-radiative relaxation pathways such as heat due to vibration or collision with the surrounding.

3.3.2Effect of CNP concentration

It was observed that the fluorescence of the CNPs recorded was concentration dependent. The intensity increased linearly with increasing amount of CNPs dispersed in per weight of deionized water at low concentration but showed a slight decrease with a further increase of the concentration. At high contentration of CNPs in water, the CNPs were subjected to Brownian motion, where random collision of the CNPs increased surface interactions among them, promoting effective fluorescence energy transfer that led to self-quenching of the fluorescence. In this study, the amount of CNPs was adjusted to be 2.0%(v/v) in water to achieve the highest intensity for the fluorescence within the scale of the spectrofluorometer. Further addition of CNPs to the solution has caused the fluorescence to exceed the analytical scale.

The total volume of the CNP solution collected from the carbonization of 0.200 g of dried rice husk was approximately 20 mL. The direct yield of CNPs in mass was not performed. It was found that the total stock of CNPs isolated can perform at least 330 sensing analyse, conresponding to 1 650 analyse for1 g of dried rice husk.

3.3.3Interfacial phenomena with metal ions

This study has successfully converted waste rice husks into fluorescent CNPs and identified the optimum synthesis conditions to obtain the best fluorescence. It will be of added value if the fluorescent CNPs can be used for a real application rather than just being an optically unique nanomaterial. For this purpose, the interfacial phenomena of the CNPs with metal ions were investigated to understand the effect of such interactions on the physical property of the CNPs. Metal ions were chosen as candidate analytes since they are positively charged with high possibility to interact with the oxidized surface of the CNPs. Besides, metal ions are of major concern due to their presence and play important roles in biological system and the environmental. The selected metal ions were those commonly found in the environment especially in water reservoir, which include Cu(II), Sn(II), Ni(II), Al(III), Co(II), Pb(II), Ag(I) and Hg(II). Some of them also pose harmful effects to living organisms.

Majority of the metal ions tested have different degrees of quenching effect on the fluorescence of the CNPs (Fig. 6), indicating that the metal ions were interacting with the colloidal CNPs at different affinities. Among the heavy metal ions tested, the Sn(II) ion had the most significant quenching effect on the fluorescence with a complete quench at high concentration of Sn(II) ion. Since this was the most effective system, the Sn(II) ion was chosen for further study. The quenching effect with increasing concentration of Sn(II) ion is shown in Fig. 7. A colour change in the solution from light brownish yellow (transparent) to milky (opaque) was clearly observed via naked eyes with increasing Sn(II) ion concentration. This visible change suggested a strong interfacial interaction between CNPs and Sn(II) ion via the formation of complex. The milky precipitate could be caused by the agglomeration of the CNPs that were bridged by the Sn(II) ions via coordination bonding. Besides, an isosbestic point was observed at around 400 nm, where all the curves intersected with similar fluorescence coefficient. This was another evidence that supported the complex formation between the Sn(II) ion and the CNPs. The initial energy transition that contributed to the fluorescence would be disturbed due to the close proximity the Sn(II) ions at the surface of CNPs, resulting in the quenching. This was also observed and reported in other similar studies[32, 33]. Besides, the oxidized carbon surface is reported to be a strong electron acceptor that will allow quenching of fluorescence by electron donors such as metal ions[25]. This could be another reason for the observed quenching since Sn(II) ions is a good reducing agent that tends to be oxidized to Sn(IV) by

Fig. 6　The quenching of fluorescence observed for the CNPs

Fig. 7　The fluorescence spectra of the CNPs excited at 358 nm

releasing electrons.

3.4Sensing application and merits

One of the possible applications for the CNPs will be as optical sensing receptors since the CNPs can be quenched when metal ions were added. In order to further develop this application, the quenching trend caused by Sn(II) ion with an increasing amount added was characterized and analyzed to evaluate its sensing performance (Fig. 7). The fluorescence intensity of the CNPs showed a stepwise decrease with the addition of Sn(II) ions each time and there was no significant shift in the emission peak that was maintained at around 439 nm. This allowed the monitoring of the sensing signals at a single wavelength, which can ease the datum interpretation. Although the quenching experience the concentration dependence trend, the relationship was not a linear one. Thus, the standard Stern-Volmer as shown by eq. (1) was adapted to model the sending signal at 439 nm.

F0/F = 1 + Ksv[C]

(1)

whereF0is the fluorescence intensity of the control without addition of Sn(II) ion,Fis the fluorescence intensity observed in the presence of Sn(II) ion,Ksvis Stern-Volmer constant and [C] is the concentration of Sn(II) ions.

The plot of the Stern-Volmer gave a linear correlation up to 6.13 mmol/L Sn(II) in water under the optimized condition. The relationship ofF= 316.61[C] + 0.9729, with a correlation coefficient,R2of 0.9965 was obtained. The repeatability of the system over 6 cycles of measurements is good with a deviation less than 3.0%. In order to evaluate the limit of detection (LOD) for Sn(II), the fluorescence of 6 blank samples containing just the fixed amount of CNPs were recorded and the standard deviation (σ)of the readings was evaluated. The LOD was calculated based on eq. (2).

LOD = 3σ/s

(2)

whereσis the standard deviation of blank (n= 6) and s is the gradient of the calibration.

The LOD was evaluated to be 18.7 μmol/L (based on 99.7% confident limit,n= 6). This low level of detection limit will enable a practical use of the CNPs as a sensing material in various areas including for a monitoring of Sn(II) ion in the environment. The potential can be further extended to food analysis especially for canned foods, which have a high risk of Sn(II) ion contamination due to the direct contact of the foods with the can. The LOD is comparable with that of standard method[34], while this probe is far easier to use, cheaper, and most importantly, lower in toxicity since the CNPs were derived from natural product. It will be less an issue of contamination or toxicity during and after the application even with food samples.

4Conclusions

This work has demonstrated an innovative and green approach in converting waste rice husks that are of low commodity value into fluorescent CNPs. The interfacial interaction of the CNPs with metal ions makes it possible to utilize the CNPs for sensing application. This can be an alternative to replace some existing fluorescent dyes or quantum dots that are less sustainable in terms of its toxicity and synthesis protocol. The CNPs, when they are optimized as a sensing material for Sn(II) ion, showed remarkable results with an analytical linear range from 18.6 μmol/L up to 6.13 μmol/L with a good repeatability.

Acknowledgements

This project was supported by the Faculty of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus under the Biotechnology Program. Partial financial support was also obtained from the Swinburne Sarawak Research Centre for Sustainable Technology under the Swinburne Sarawak Research Grant (SSRG), Grant No. 2-5509. The authors would like to acknowledge the supports by all the staff, technicians, postgraduates, and collaborators that have contributed to this study.

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Synthesis of carbon nanoparticles from waste rice husk used for the optical sensing of metal ions

Peggy Zhen Zhen Ngu1,Stephanie Pei Phing Chia1,Jessica Fung Yee Fong1,2,Sing Muk Ng1,2

(1.FacultyofEngineering,Computing,andScience,SwinburneUniversityofTechnologySarawakCampus,JalanSimpangTiga93350,Kuching,Sarawak,Malaysia；2.SwinburneSarawakResearchCentreforSustainableTechnologies,SwinburneUniversityofTechnologySarawakCampus,JalanSimpangTiga93350,Kuching,Sarawak,Malaysia)

Abstract:This work reports on a synthesis of carbon nanoparticles (CNPs) from waste rice husk by thermally-assisted carbonization in the presence of concentrated sulfuric acid. The fluorescent emmision characteristics of the CNPs, their quenching effects by metal ions and their use as a sensing material for Sn(II) ions were investigated. Results indicated that the yield of CNPs was optimized at a sulphuric acid concentration of 12 mol/L, heating temperature of 1 200 ℃ and heating time of 30 min. The sample showed a strong blue luminescence in water with a maximum emission at 439 nm. The fluorescence can be quenched by adding various metal ions by the formation of complexes between the metal ions and surface of the CNPs. Sn(II) ions had the most significant quenching effect on the fluorescence of the CNPs, which is concentration-dependent. The concentration dependent quenching was linearized with the Stern-Volmer equation, and showed a linear response up to a Sn(II) concentration of 6.13 mmol/L. The limit of detection for Sn(II) ions is 18.7 μmol/L with good repeatability.

Keywords:Carbon nanoparticles; Fluorescence; Quenching; Sensing; Metal ions

文章编号：1007-8827(2016)02-0135-09

中图分类号:TQ127.1+1

文献标识码:A

通讯作者：Sing Muk Ng. E-mail: smng@swinburne.edu.my

Corresponding author:Sing Muk Ng. E-mail: smng@swinburne.edu.my

DOI:10.1016/S1872-5805(16)60008-2