Multicolor Functional Carbon Dots via One-Step Refluxing Synthesis

Feb 13, 2017 - Carbon dots are admirable fluorescent nanomaterials due to their low cost, high photostability, excellent biocompatibility, and environ...
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Multicolor Functional Carbon Dots via One-Step Refluxing Synthesis Ting-Yi Wang, Chong-You Chen, Chang-Ming Wang, Ying Zi Tan, and Wei-Ssu Liao ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00607 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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Multicolor Functional Carbon Dots via One-Step Refluxing Synthesis Ting-Yi Wang†, Chong-You Chen†, Chang-Ming Wang, Ying Zi Tan, and Wei-Ssu Liao* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

ABSTRACT: Carbon dots are admirable fluorescent nanomaterials due to their low cost, high photostability, excellent biocompatibility, and being environment friendly. Most conventional carbon dot fabrication approaches produce single-colored fluorescent material in the preparation process, different methods are therefore required to synthesize distinct carbon dots for specific optical applications. In this study, carbon dots carrying different emission colors are prepared through a one-step refluxing process. The emission of these materials can be well-tuned by sodium hydroxide content in the precursor solution. The produced carbon dots are used as sensing probes based on spectrofluorometric inner filter effect for target molecules detection. Three sensing categories that combine carbon dots and inner filter effect are demonstrated, including direct, metal nanoparticle-assisted, and enzymatic reaction-supported detection. Caffeine, melamine, and fenitrothion are selected as targets to demonstrate the strategies, respectively. These multi-functional carbon dots based sensors achieve comparable sensitivity toward analytes with a much more convenient preparation route. KEYWORDS:

Carbon

dots,

refluxing

synthesis,

Fluorescent nanomaterials, such as quantum dots, metal nanoclusters, and carbon dots (CDs) are important fluorescent probes that can be utilized to detect various type of target molecules.1-4 The sensing mechanism usually involves fluorescence quenching processes initiated in between analytes and nanomaterials, such as cyanide-triggered etching or metallophilic interaction.5 For instance, gold nanoclusters can be used to detect cyanide (CN-) by etching-induced fluorescence quenching due to their strong interaction. 5 On the other hand, silver nanoclusters can sense mercury ion (Hg2+) via formation of strong metallophilic bonds between Hg2+ (5d10) and Ag+ (4d10) d10 centers which quenches their fluorescence signal.6 Relying on surface modification and functionalization, 7 the sensing capabilities of these materials can be further expanded.3, 8-9 For example, quantum dots can be used for biological labelling through conjugation with organic molecules, or to sense a specific target by fluorescence resonance energy transfer via aptamer functionalization.9-10 However, complicated synthesis procedure and harmful precursor usage, 9 such as cadmium or selenium, are often required in fabrication of metal quantum dots.10-11 Although noble metal nanoclusters conduct extensive application such as fluorescent probes, their preparation procedure is considerably more expensive and toxic.3-5 Carbon dots are environment-friendly fluorescent nanomaterials with high photostability.12 Comparing to quantum dots and noble metal nanoclusters, carbon dots benefit from their low cost and fabrication simplicity. Numerous methods have been reported for efficient synthesis of this material, such as calcination,13 ultrasonication,14 electrochemical oxidation,15 microwave-assisted methods,16-17 as well as hydrothermal synthesis.18-20 Amino acids, carbohydrates, or other carbon-rich

multicolor,

fluorometric

sensors,

inner

filter

effect

precursor compounds are commonly used as source materials to create carbon dots through dehydration, polymerization, carbonization or other processes.19 Carbon dots possess attractive merits such as unique tunable emission optical property,19-21 and are widely used for multicolor patterning,22 biosystems,23-24 invisible inks,25 catalysts,26 and sensing probes.2, 27 For example, they can act as fluorescent sensing probes for metal ion detection by complexation through their strong binding and chelating affinity due to abundant surface carboxyl, amine, and hydroxyl groups. 23-24 Based on the effective coordination interaction between metal ions and carbon dots, the fluorescence emission of this material are diminished by static or dynamic quenching processes. 2829 However, these metal ion detections strongly rely on carbon dot surface functionality, and specific metal ion sensing usually requires a corresponding synthesis method.2, 28 Surface modification is a useful approach to expand the analytical application and target detection of carbon dots. 7, 30-33 The produced probes can incorporate with non-radiative fluorescence quenching strategies for target recognition. Common approaches, including inner filter effect (IFE), fluorescence resonance energy transfer (FRET), and photoinduced electron transfer (PET), are all compatible with this material. 34-38 For instance, carbon dots can be modified with organic dyes to detect hydrogen sulfide (H2S) via dye reduction induced FRET effect.30 In addition, ethylenediamine modified carbon dots can detect silver nanoparticles via particle aggregation induced IFE phenomenon.39 Among these spectrofluorometric phenomena, it should be noted that the efficiency of FRET and PET-based strategies are highly dependent on the distance between fluorophores and receptors,40-43 while IFE systems are not bound to this

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restriction.44-46 Two major types of fluorescence quenching phenomena can be classified in an IFE-based strategy. In the conventional IFE mechanism, the fluorophore’s emission band overlaps with absorber absorption, causing fluorophore emission intensity to decrease due to non-radiative energy conversion. Alternatively, the spectra of fluorophore excitation and absorber absorption are coincident to induce irradiation shielding. This leads to incident radiation competition between fluorophore and absorber, resulting in highly reduced fluorescence emission intensity.11, 35-36 Therefore, IFE-based strategies do not need covalent linkage while offering high sensitivity, considerable flexibility and simplicity, comparing to other fluorescence quenching approaches.35-36 In this study, a straightforward and one-step refluxing process for multicolor carbon dots synthesis is demonstrated. With the control of base content in the precursor solution, carbon dots exhibiting different fluorescent colors are prepared. The carbon dots act as fluorophores, and in conjunction with complementary absorber can efficiently initiate the irradiation shielding process through IFE. The reduction of carbon dot fluorescence intensity is in turn applied to quantify target analytes via direct or indirect IFE. Carbon dots of three different colors, including blue, green, and yellow, are prepared by our one-step refluxing method. Incorporating with different IFE categories, approaches including direct, metal nanoparticle-assisted, and enzymatic reaction-supported fluorescence signal quenching are utilized to detect caffeine, melamine, and fenitrothion, respectively. EXPERIMENTAL SECTION Chemicals. D-(+)-galactose, fenitrothion, 4-nitrophenyl phosphate di(tris) (NPP), 4-nitrophenol (NTP), alkaline phosphatase (ALP), caffeine, melamine, quinine sulfate, fluorescein isothiocyanate (FITC), silver nitrate (AgNO3), trisodium citrate dihydrate (Na3C6H5O7∙2H2O), and sodium borohydride (NaBH4) were purchased from Sigma Aldrich (St. Louis, MO, USA). L-cysteine was purchased from Acros Organics (Geel, Belgium). Sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium hydroxide, and magnesium sulfate (MgSO4) were obtained from SHOWA (Tokyo, Japan). Ultrapure water (>18.2 MΩ · cm) generated from ELGA PURELAB classic system (Taipei, Taiwan) was used throughout the experiments. Instruments. High-resolution transmission electron microscopy (HRTEM) images of carbon dots were obtained employing Philips Tecnai F20 G2 field-emission system operating with 200 kV electron beams. The infrared transmittance spectrum was acquired from Perkin Elmer 2000 FT-IR spectrometer with KBr pellets. Dynamic light scattering (DLS) measurements were obtained with Malvern Zetasizer Nano S instrument. Elemental analysis (EA) was accomplished by the elementar Vario EL cube. Absorption and fluorescence spectra of carbon dots solution were measured by Thermo Evolution UV-220 spectrophotometer and Edinburgh FS920 fluorometer, respectively. Fluorescent carbon dots synthesis. Precursor solutions (20 mL) containing 120 mM L-cysteine, 10 mM D-(+)-galactose, and various concentrations of NaOH were refluxed at 80 oC with constant vigorous stirring for 24 hours. With the use of 0.4, 0.2, and 0.1 M NaOH, blue, green, and yellow carbon dots were produced, respectively. The solutions were cooled down

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to room temperature and dialyzed against ultrapure water through dialysis membranes (Spectrumlabs, Rancho Dominguez, CA, USA) at 3500 molecular weight cutoff for one day. Thereafter, the dialysis-purified solutions were freeze-dried in a lyophilizer for 2 days to collect the carbon dot product powders. These carbon dots were then suspended in the PBS buffer (10 mM, pH 9.0) and stored at -20 oC in dark before further use. The synthesis of different color carbon dots and products characterization have been repeated for at least three times independently to verify the batch-to-batch reproducibility. Silver nanoparticles synthesis. A round bottom flask containing 18.9 mL ultrapure water was pre-heated in a 30 oC water bath. 50 μL of both 0.1 M AgNO3 and 0.1 M Na3C6H5O7 aqueous solutions were then added, and the solution was kept stirred for 3 minutes. Thereafter, 1 mL of 0.01 M NaBH4 aqueous solution was slowly added into the mixture with vigorous stirring for two hours in the dark. This resulted in a 6.86 nM silver nanoparticle solution with extinction coefficient of 5.56 x 108.47 The HRTEM images indicate that size of silver nanoparticles to be 9.5 ± 1.5 nm (100 counts), as shown in Figure S1. The silver nanoparticle solution was stored in dark at room temperature before further use. Direct detection of caffeine with blue carbon dots (BCDs). Blue carbon dots were synthesized by the aforementioned one-step refluxing method. Caffeine solutions of various concentrations were independently prepared in a pH 9.0 PBS buffer containing 0.02 mg/mL BCDs, and diluted to a final volume of 0.5 mL for fluorescence spectra measurements. The concentration of caffeine was determined by fluorescence intensity measurement at 400 nm under excitation wavelength of 280 nm. Direct detection of caffeine in complex samples with blue carbon dots (BCDs). To assess the possibility of carbon dots based detection in complex matrix, the system is applied to detect caffeine in Sprite™. pH 9.0 PBS buffer was used to dilute Sprite ten-fold, and caffeine was spiked into the solution to obtain a final concentration of 50 μM. Finally, BCDs were added into the solution and fluorescence spectra were taken to calculate the recovery rate. Detection of melamine with green carbon dots (GCDs) and silver nanoparticles. Melamine solutions of various concentrations were prepared in 0.5 mL of 10 mM pH 8.0 PBS buffer that contains 2.4 nM silver nanoparticles. The presence of melamine caused silver nanoparticles to aggregate after one hour of reaction, and the solution color changed from pale yellow to brown. This mixture was then centrifuged at 2000 rpm for 20 minutes to remove aggregated particles, and the supernatant part was collected. 10 μL 0.8 mg/mL GCDs was thereafter added into 390 μL of this supernatant solution before fluorescence spectra measurements. The concentration of melamine was determined by fluorescence intensity measurement at 495 nm under excitation wavelength of 395 nm. Detection of melamine in complex samples with green carbon dots (GCDs) and silver nanoparticles. The effectiveness of this system was testified in complex sample of powdered milk. 0.02 g commercially available milk powder was dissolved in 10 mL ultrapure water, centrifuged at 10000 rpm for 30 minutes to remove colloidal species, then filtered with a 0.22 μm syringe filter. After spiking 15 μM melamine

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into the solution, the sample was tested with the aforementioned silver nanoparticles-assisted detection method. Detection of fenitrothion with yellow carbon dots (YCDs) associated ALP enzymatic system. 0.1 mL fenitrothion solution of various concentrations were obtained by adding fenitrothion into a mixture containing ALP (1 U/mL), MgSO4 (5 μM), and NPP (1 mM) in pH 9.0 PBS buffer. After incubation in water bath at 30 oC for one hour, the solution turned yellow. 0.4 mL YCDs (0.25 mg/mL) were then added into this solution, and the mixture was ready for fluorescence spectra measurements. The fenitrothion concentration determination and analyte selectivity tests were operated by fluorescence intensity measurement at 550 nm under excitation wavelength of 405 nm. Quantum yield measurements. Quantum yields of carbon dots were determined by the following equation:19 Φcarbon dots ＝Φreference ×(

η2 mcarbon dots dots )×( carbon ) mreference η2reference

where Φ, m and η represent the quantum yield, slope of integrated fluorescence intensity, and refractive index of solvent, respectively. Fluorescein isothiocyanate (Φ = 0.95 in 0.1 M NaOH) and quinine sulfate (Φ = 0.54 in 0.1 M H 2SO4) were chosen as references. ΦCDs was calculated by comparing the integrated fluorescence intensity of carbon dots with references. Cuvettes with 1 cm optical path length were utilized to measure sample absorption. The absorbance of carbon dots and references were both set to be under 0.1 at their excitation wavelength in order to minimize the re-absorption effect.2, 17 Detection of fenitrothion in complex samples with yellow carbon dots (YCDs) associated ALP enzymatic system. To assess the method effectiveness on complex samples, the determination of fenitrothion amount in orange peels was applied. 0.1 g of a freshly cut orange peel was first immersed in 10 mL methanol for two hours, and 30 μM fenitrothion was spiked into the solution after removal of the orange peel. The solution was then added into the ALP, MgSO4, and NPP mixture as described previously, and incubated in the 30 oC water bath. Finally, 0.4 mL of YCDs (0.25 mg/mL) was introduced into the solution for fluorescent emission spectra measurements. RESULTS AND DISCUSSION One-step refluxing synthesis of fluorescent carbon dots. In conventional approaches, hydrothermal methods can produce highly photoluminescent carbon dots, but high temperature operation conditions and complicated reaction pathways are required to initiate the carbonization process.2, 13 On the other hand, the microwave-assisted method is a comparably easier way to perform, but a specialized reaction equipment is necessary.16-17 Alternatively, the electrochemical oxidation can be also carried out with three-electrode systems to produce carbon dots, but the synthesis condition is restricted to nitrogen ambient.48 In addition to aforementioned synthesis criteria, the single color material production via individual fabrication process limits the application capabilities of these materials.

Scheme 1. Schematic demonstration of the multi-color carbon dot one-step refluxing synthesis. Herein, a straightforward refluxing method to prepare carbon dots from carbohydrates and amino acids at moderate temperature by the Maillard reaction is demonstrated.49 Maillard reaction is a widely used condensation reaction happening in between reduced saccharide and amino acids at high temperature under moderate alkaline conditions. This strategy can be applied to fabricate carbon dots with a diverse of applications, but typical approaches can produce only single color emission carbon dots.49-50 In this work, multi-color carbon dots are produced by a one-step hydrothermal refluxing process. Cysteine and galactose are chosen as source materials and the reaction is performed in a sodium hydroxide solution. 49, 51 At the beginning of reaction, precursors presumably transform into Maillard reaction products (MRPs).51 When MRPs dehydrate, formation of furan-like compounds appear in the solution. Condensation and polymerization then take place to link furan-like molecules with each other,52 and finally the condensated species transform into various soluble polymeric fragments.19 Once these polymeric species reach the critical supersaturation point, carbonization and aromatization arise, resulting in many single nuclear burst of aromatic clusters. The as-formed nuclei then undergoes isotropic growth to small particles in the solution, finally transforming into carbon dots (Scheme 1).16 It is important to note that sodium hydroxide can effectively accelerate the carbonization process in the carbon dot synthesis.49, 51 We anticipate sodium hydroxide to be the key in influencing isotropic growth and transformation of different-colored carbon dots. The use of lower sodium hydroxide concentration in reaction is therefore expected to slow down the formation of carbon nuclei, resulting in larger carbon dots development due to slower isotropic growth. Carbon dots synthesized under different sodium hydroxide concentration through our one-step refluxing process are characterized as shown in Figure 1. The HRTEM images (Fig 1A-1C) reveal the lattice structure of synthesized CDs under different sodium hydroxide concentration conditions. The sizes of the CDs are determined by the dynamic light scattering (DLS) technique (Fig 1D), revealing particle size of BCDs, GCDs, and YCDs to be 3.3 ± 0.5 nm, 6.1 ± 0.6 nm, and 10.4 ± 1.2 nm, respectively. It is clear that the size of synthesized carbon dots increase when sodium hydroxide concentration is reduced in the reaction. The production of carbon dots with different diameters is therefore possible through straightforward experimental condition modulation in the same process, which is difficult to achieve with other carbon dots synthesizing techniques.

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stretching.56 Furthermore, stretching modes of C-O-C bonds at 1300 cm-1 (asymmetric) and 1200 cm-1 (symmetric) are also observed, indicating the presence of furan-like structures.56 Due to these characteristic IR vibrational modes, starting materials are believed to transform into carbon dots which are composed by MRPs. Elemental Analysis (%) C

H

O

N

S

BCD

21.4

4.9

57.4

5.6

10.7

GCD

23.4

4.9

51.9

6.9

12.9

YCD

25.0

4.8

48.8

6.7

14.7

Table 1. Elemental analysis of blue, green, and yellow carbon dots. Figure 1. HRTEM images of carbon dots synthesized under precursor solutions containing (A) 0.4 M, (B) 0.2 M, (C) 0.1 M NaOH. Scale bars of (A) and (B): 5 nm; (C): 20 nm. (The inset image scale bars are all 2 nm.) (D) The corresponding carbon dot size distribution histograms obtained through dynamic light scattering measurements. The blue, green, and yellow columns represent the hydrodynamic size distribution of BCDs (1.7-3.5 nm), GCDs (4.8-7.5 nm), and YCDs (8.711.7 nm), respectively. Fluorescent carbon dots characterization. Carbon dots are products of carbon-rich source materials after carbonization isotropic growth, which are highly crystalline and consist obvious parallel lattice crystal planes.2, 53-54 As shown in Figure 1, our blue and yellow carbon dots exhibit a lattice spacing (d-spacing) of around 0.18 and 0.22 nm, respectively, which is similar to the (100) X-ray Diffraction (XRD) pattern of graphene.53 On the other hand, our green carbon dots carry a d-spacing of 0.32 nm, which corresponds to (002) XRD pattern of graphene.54 In other words, our hydrothermal refluxing method produces carbon nuclei that crystallizes into carbon dots, and their characteristic d-spacing is identical to different graphene XRD patterns. Since the carbon dot suspension solution undergoes a dialysis process following freeze-drying, unreacted source materials are removed before product powder collection. Different material characterization instruments can therefore be utilized to identify remaining functional groups on the synthesized carbon dots. Due to cysteine precursor thiol groups, the sulfur signal is recognizable by the energy dispersive X-ray spectroscopy (EDS) (Figure S2). In order to analyze the component of carbon dots, elemental analysis is used to quantify carbon dots composition (Table 1). These carbon dots are verified to be composed of carbon, hydrogen, oxygen, nitrogen, and sulfur, which are main elements of the organic precursors. In FT-IR characterization (Figure S3), distinct peaks present at 3430 cm-1 indicate the stretching vibrations of O-H and N-H bonds.19 The major carbon source, i.e. cysteine, can also be recognized by the symmetric bending of primary amines (-NH2) and stretching vibrations of thiol groups at 1493 cm-1 and 2600 cm-1, respectively.2, 55 In addition, the peaks at around 1587 cm-1 and 1400 cm-1 are attributed to asymmetric and symmetric carboxyl group (-COOH)

Optical properties of fluorescent carbon dots. Due to the small size of synthesized carbon dots, UV-vis and fluorescence spectrum can be used to illustrate the unique optical properties of these materials. As they are composed of MRPs, absorption bands of π→π* and n→π* transitions in the ultraviolet region at 280-320 nm are clearly visible for all three carbon dots, as shown in Figure 2A.2, 56 Due to quantum confinement effect, the fluorescent property of carbon dots depends on their size,57-58 which is in turn dependent on the sodium hydroxide content in the reaction (Figure 1D). Therefore, we can deduct the relationship between NaOH concentration and resulting emitted color of carbon dots – higher NaOH concentration leads to longer wavelength fluorescence emission (Figure 2B). However, only emission intensity decreases can be observed when different excitation wavelengths were employed, not emission wavelength shifts. The result reveals that our BCDs, GCDs, and YCDs do not have excitation-dependent property, which can be attributed to carbon dots surface uniformity (Figure S4). 56 It is also important to note that the quantum yields are calculated to be 0.02 for BCDs by comparing to quinine sulfate. The fluorescein isothiocyanate acts as reference with GCDs and YCDs, and their quantum yields are estimated to be 0.34 and 0.76, respectively (Figure S5). The quantum yields of fabricated multi-color carbon dots are comparable with similar materials produced by other techniques.17, 57

Figure 2. (A) UV-vis absorption spectra of BCD, GCD, and YCD aqueous solutions. The inset is a photograph representing BCDs, GCDs, and YCDs under UV lamp (365 nm) illumination. (B) Fluorescence emission spectra of BCD, GCD, and YCD aqueous solutions under 300, 390, and 395 nm of excita-

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tion, respectively. The blue, green, and yellow lines represent the corresponding carbon dot emission colors. Inner filter effect based carbon dot sensing platforms. In an IFE system, the wavelength of fluorophore excitation coincides with absorber absorption, this irradiation shielding results in incident light competing and induces emission intensity decline. This phenomenon can be utilized for analytical sensing platforms with proper selection of fluorescent material and corresponding absorber pairs. Conventionally, quantum dots and gold nanoclusters can act as a fluorophore in the IFE related strategy, but have environmental concerns due to the toxic heavy metal usage.11, 36, 47 On the other hand, fluorescent carbon dots can play similar roles but without aforementioned concerns. Herein, we demonstrate the capability of prepared multicolor functional fluorescent carbon dots for target molecules sensing based on this approach. Three different platform designs are studied as follows:

Figure 3. (A) Spectra of caffeine absorption (black curve), BCDs excitation (dashed blue curve), and BCDs emission (blue curve). (B) Fluorescence emission spectra of BCDs in the presence of (a) to (l): 0, 1, 5, 10, 25, 50, 75, 100, 150, 200, 250, 300 μM caffeine. (C) Calibration curve of caffeine concentration versus relative BCDs fluorescence emission intensity at 400 nm under 280 nm of excitation. Inset shows the linear range from 1 to 75 μM. F0 and F are the BCD fluorescence emission intensity before and after caffeine addition, respectively. (N = 3)

Scheme 2. Schematic illustration of the direct fluorophore-absorber competition IFE process for caffeine detection by the use of BCDs. Direct fluorophore-absorber competition. In this strategy, a proper pair with matching fluorophore excitation and absorber absorption is chosen to initiate the inner filter effect. As shown in Figure 3A, blue carbon dot (BCD)’s excitation wavelength is close to caffeine’s characteristic absorption band with a small gap of 26 nm at the maximum wavelength. In order to obtain effective irradiation shielding, our BCDs are excited by a 280 nm incident light at the intersection point of caffeine absorbance and BCDs excitation spectra, which leads to a 400 nm fluorescence emission. This BCD-caffeine pair is therefore expected to initiate an effective IFE, as shown in Scheme 2. It was found that caffeine can compete with BCD, which leads to decreased fluorescence emission intensity when more caffeine is introduced (Figure 3B). The relationship is linear in the range of 1 μM to 75 μM with the correlation function [(F0-F)/F0] = 0.007 [caffeine] + 0.0163 (R2 = 0.9946), as shown in Figure 3C, where F0 and F respectively represent the fluorescence emission intensity of BCD before and after caffeine addition. It should be noted that the BCD-caffeine IFE pair reaches minimum detectable concentration of 1 μM and maintains a broad detection range for caffeine sensing. This IFE-based approach is more sensitive than other fluorometric caffeine sensors using organic dyes.59 Since the analyte detection in this approach is based on irradiation shielding, interference species with an absorption band in this wavelength range cannot be well distinguished.

Scheme 3. Schematic illustration of the metal nanoparticle-assisted IFE process for melamine detection by the use of GCDs. Metal nanoparticle-assisted IFE process. In order to expand the carbon dot application to targets with little spectral overlap, an alternative nanoparticle-assisted IFE strategy is designed. Melamine is a nitrogen-rich organic molecule with maximum absorbance at 208 nm in UV region, while green carbon dots (GCDs) have maximum excitation at 395 nm. Effective irradiation shielding does not occur naturally if these two materials are selected as an IFE pair (Figure 4A). Silver nanoparticle is therefore chosen as an assisting absorber to achieve indirect melamine detection. For demonstration, citrate-stabilized silver nanoparticle is used as the absorber. The sharp extinction peak of silver nanoparticle at 395 nm due to localized surface plasmon resonance matches the excitation wavelength of GCDs. The silver nanoparticles can therefore

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act as an effective irradiation shielding intermediate between fluorescent materials and melamine molecules, as displayed in Scheme 3. When citrate-protected silver nanoparticles are mixed with melamine, the solution color turns brown from pale yellow, and a shoulder peak at around 450-500 nm appears in the UV-vis spectrum.60 This is attributed to the aggregation of silver nanoparticles when melamine is present. The negatively charged citrate protecting groups on nanoparticle surface attract positively charged amino groups and the 1,3,5-triazine ring of melamine via electrostatic interaction in a pH 8.0 PBS buffer, whereas the acid association constant (pKa) of melamine is 8.95.60 Furthermore, melamine also offers intermolecular NH-N hydrogen bonding sites because it’s a nitrogenrich compound.47, 60 Both factors result in nanoparticle aggregation and solution color change when melamine is introduced.

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approach is determined to be 0.1-20 μM with the calibration curve [(F-F0’)/(F0-F0’)] = 0.0241 [melamine] - 0.6706 (R2 = 0.9924), as shown in Figure 4D. It should be noted that the achieved minimum detectable concentration of 0.1 μM is comparable with previous research.61 Several interference analogue species were also tested with this approach. The results indicate that these analogues do not cause silver nanoparticles to aggregate and no obvious IFE phenomenon was observed (Figure S6). It is worthy to note that melamine molecule shows significant influence on aggregation of silver nanoparticles and the following IFE process, owing to chemical structure and intermolecular hydrogen bonding. We believe that this nanoparticle-assisted strategy can expand the diversity of perspective analytes that cannot induce IFE directly.

Scheme 4. Schematic illustration of the enzymatic reaction-supported IFE process for fenitrothion detection by the use of YCDs. Figure 4. (A) Spectra of melamine absorption (orange curve), AgNPs absorption (black curve), GCDs excitation (dashed green curve), and GCDs emission (green curve). (B) UV-vis absorption spectra of centrifuged AgNP solution supernatants after melamine addition. (a) to (l) represents melamine concentration of 0, 0.1, 0.5, 1.0, 2.5, 5.0, 7.5, 10, 15, 20, 30, 50 μM. (C) Fluorescence emission spectra of GCDs with different concentrations of melamine. (D) Calibration curve of melamine concentration versus relative GCDs fluorescence emission intensity at 495 nm under 395 nm of excitation. Inset shows the linear range from 0.1 to 20 μM. F0 is the initial GCD fluorescence emission intensity. F0’ and F refer to the GCD fluorescence emission intensity in presence of 0 μM and other concentrations of melamine. (N = 3) The aggregated silver nanoparticles are removed by centrifugation while the remaining dispersed nanoparticles in the supernatant solution is used as the IFE absorber. Since a portion of silver nanoparticles is eliminated, the characteristic absorption peak density of the supernatant solution at 395 nm decreases as melamine concentration is raised (Figure 4B). On the other hand, higher concentration of melamine results in stronger fluorescence emission of GCDs, due to less silver nanoparticle triggered IFE (Figure 4C). Although the silver nanoparticle extinction reduction could simply be used to point out melamine’s existence, sensitive fluorescence detection is adopted in this strategy to expand perspective analyte varieties. The linear detection range of melamine under this

Enzymatic reaction-supported IFE process. In addition to metal nanoparticles, biological species such as enzymes can also incorporate with the biocompatible carbon dots. For demonstration, the fluorescent carbon dots are combined with an enzymatic reaction to detect an organophosphorus pesticide, fenitrothion. In Figure 5A, the hydrolysis of 4nitrophenyl phosphate (NPP) molecule to 4-nitrophenol (NTP) leads to the absorption band shift from 310 nm to 405 nm, while NTP’s absorption band coincides with the excitation wavelength of yellow carbon dots (YCDs). This spectra overlapping can therefore be used to initiate a similar irradiation shielding process as described above. Since this hydrolysis reaction is catalyzed by alkaline phosphatase (ALP), the YCD emission intensity at 550 nm can be modulated via ALP activity. Furthermore, fenitrothion is able to inhibit ALP’s activity, consequently resulting in decreased NPP hydrolysis to NTP. 6264 Therefore, the YCDs can accordingly incorporate with this enzymatic reaction to monitor the existence of fenitrothion pesticide as displayed in Scheme 4. After the addition of fenitrothion, Figure 5B represents that higher concentration of fenitrothion diminish NPP hydrolysis, resulting in greater NPP absorbance than NTP, and vice versa. On the other hand, stronger YCD emission intensity indicates more ALP inhibition under higher fenitrothion concentration (Figure 5C). The fluorescence intensity change of YCDs can therefore be used to determine the introduced fenitrothion quantity at a fixed reaction time. Detection of fenitrothion

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through this design presents a linear range between 1-50 μM with a calibration curve [(F-F0’)/(F0-F0’)] = 0.0042 [fenitrothion] - 0.8347 (R2 = 0.9967), as shown in Figure 5D. Organophosphorus pesticide fenitrothion sensing has reached minimum detectable concentration of 1 μM through this YCD-NTP IFE pair. As demonstrated, this enzymatic reaction supported IFE-based strategy provides a reasonable detection range to quantify the amount of organophosphorus pesticide fenitrothion.

In order to test whether the above-mentioned three strategies could detect targets in complex samples, recovery experiments are employed for demonstration. By standard addition method, caffeine, melamine, and fenitrothion were respectively spiked into the prepared samples of Sprite™, milk powder, and orange peels. As shown in Table 2, direct detection of caffeine exhibits a recovery rate of 100.55%, while metal nanoparticle-assisted detection of melamine and enzyme-supported determination of fenitrothion result in recovery rates of 99.44% and 102.46%, respectively. The high recovery rate with low relative standard deviation in the range of 0.7-3.90% indicates the excellent stability and reproducibility of the presented methods and their good potential for targets detection in real samples.

Sample

Target added (μM)

Target found (μM)

Recovery (%)

RSD (%)

Sprite

50

50.27 ± 0.35

100.55

0.71

Powder milk

15

14.92 ± 0.58

99.44

3.90

Orange peel

30

30.73 ± 0.45

102.46

1.47

Table 2. Determination of caffeine, melamine, and fenitrothion in complex samples. (N = 3) Figure 5. (A) Spectra of NTP absorption (black curve), YCDs excitation (dashed yellow curve), and YCDs emission (yellow curve). (B) UV-vis absorption spectrum demonstrating NPP hydrolysis (305 nm) and NTP (405 nm) formation with the addition of fenitrothion. (a) to (k): 0, 1, 2, 5, 10, 20, 30, 40, 50, 60, 80 μM fenitrothion. (C) Fluorescence emission spectra of YCDs after adding (a) to (k): 0, 1, 2, 5, 10, 20, 30, 40, 50, 60, 80 μM of fenitrothion. (D) Calibration curve of fenitrothion concentration versus relative YCDs fluorescence emission intensity at 550 nm under 405 nm of excitation. Inset shows the linear range from 1 to 50 μM. F0 is the initial YCD fluorescence emission intensity. F0’ and F refer to the YCD fluorescence emission intensity in presence of 0 μM and other concentrations of fenitrothion. (N = 3) Analyte selectivity it is an important aspect in a pesticide sensor, the performance of this fenitrothion detection is therefore tested in the presence of various potential interferences such as sugars and organic acids from typical real samples. 65 The interference efficiency (IE) is defined by the equation:66 Interference efficiency (%) ＝

Fwithout － Fwith × 100 % Fwithout － F0

in which Fwithout and Fwith respectively refers to the YCD emission intensity in absence and in presence of interferences, and F0 refers to the initial YCD emission intensity. It is found that fenitrothion greatly inhibits the ALP activity in this sensing system, but not other common organic compounds, such as ascorbic acid, malic acid, or glucose (Figure S7). Results reveal that this design provides an excellent specificity for fenitrothion with little interference. This strategy can expand the capability of carbon dot materials in designing biosensors and the conjunction with other biological species for target detection.

CONCLUSIONS A one-step refluxing synthesis method to prepare carbon dots with different emission colors, including blue, green, and yellow, is demonstrated. Maillard reaction is utilized to convert carbohydrates and amino acids into fluorescent carbon dots through a straightforward hydrothermal refluxing route. Although the refluxing approach requires overnight incubation, the size and optical properties of synthesized carbon dots can be easily well-tuned by adjusting the sodium hydroxide content in the reaction. To demonstrate practical application of the multicolor carbon dots, they are employed as analytical probes to detect various target molecules through spectrofluorometric inner filter effect. Three IFE-based strategies, including direct fluorophore-absorber competition, metal nanoparticle-assisted IFE process, and enzymatic reaction-supported IFE process are performed to present potential feasibility of multicolor nanosensor developments. Similar or lower minimum detectable concentrations of caffeine, melamine, and fenitrothion comparing to other sophisticated strategies are achieved. Although direct detection with carbon dots possesses limited selectivity due to the mechanism of IFE, the other two detection routes offer excellent selectivity over target analogs even in real samples. Due to fabrication process simplicity and sensing strategy flexibility, this approach expands the capability of fluorescent carbon dots and their joint analytical application with other materials, including biosensors and in vivo detections. ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website. Elemental analysis, EDS spectra, quantum yield measurements, FT-IR spectra, interference efficiency. (PDF) AUTHOR INFORMATION Corresponding Authors *Phone: +886-2-33668712. E-mail: [email protected] Author Contributions †T.-Y. W. and C.-Y. C. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Taiwan Ministry of Science and Technology (NSC 102-2113-M-002-012-MY2 and MOST 104-2113-M-002-010-MY2). The authors would like to thank Profs. Huan-Tsung Chang and Pi-Tai Chou at National Taiwan University for fluorescence spectrometer support; Profs. Tsung-Shing Wang at National Taiwan University, and Paul S. Weiss at University of California, Los Angeles for helpful discussions. REFERENCES 1. Brahim, N. B.; Mohamed, N. B. H.; Echabaane, M.; Haouari, M.; Chaâbane, R. B.; Negrerie, M.; Ouada, H. B., Thioglycerol -functionalized CdSe quantum dots detecting cadmium ions. Sens. Actuator B-Chem. 2015, 220, 1346-1353. 2. Li, C.-L.; Huang, C.-C.; Periasamy, A. P.; Roy, P.; Wu, W.C.; Hsu, C.-L.; Chang, H.-T., Synthesis of photoluminescent carbon dots for the detection of cobalt ions. RSC Adv. 2015, 5, 2285-2291. 3. Song, X.-R.; Goswami, N.; Yang, H.-H.; Xie, J., Functionalization of metal nanoclusters for biomedical applications. Analyst 2016, 141, 3126-3140. 4. Adegoke, O.; Seo, M.-W.; Kato, T.; Kawahito, S.; Park, E. Y., An ultrasensitive SiO2-encapsulated alloyed CdZnSeS quantum dot-molecular beacon nanobiosensor for norovirus. Biosens. Bioelectron. 2016, 86, 135-142. 5. Liu, Y.; Ai, K.; Cheng, X.; Huo, L.; Lu, L., Goldnanocluster -based fluorescent sensors for highly sensitive and selective detection of cyanide in water. Adv. Funct. Mater. 2010, 20, 951-956. 6. Yuan, X.; Yeow, T. J.; Zhang, Q.; Lee, J. Y.; Xie, J., Highly luminescent Ag+ nanoclusters for Hg2+ ion detection. Nanoscale 2012, 4, 1968-1971. 7. Zhou, J.; Lin, P.; Ma, J.; Shan, X.; Feng, H.; Chen, C.; Chen, J.; Qian, Z., Facile synthesis of halogenated carbon quantum dots as an important intermediate for surface modification. RSC Adv. 2013, 3, 9625-9628. 8. Ruedas-Rama, M. J.; Hall, E. A. H., Azamacrocycle activated quantum dot for zinc ion detection. Anal. Chem. 2008, 80, 8260-8268. 9. Zhang, C.-y.; Johnson, L. W., Single quantum-dot-based

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