RSC Advances

PAPER

Synthesis of pho

aDepartment of Chemistry, National Taiwan

Taipei 10617, Taiwan. E-mail: changht@

+886-2-33661171bDepartment of Bioscience and Biotechnolog

Pei-Ning Road, Keelung, 20224, TaiwancCenter of Excellence for the Oceans, Nation

Road, Keelung, 20224, TaiwandSchool of Pharmacy, College of Pharmacy, K

80708, TaiwaneDepartment of Engineering and System S

Hsinchu, 30013, TaiwanfNano Science and Technology Program, T

Academia Sinica, Taipei, 11529, Taiwan

† Electronic supplementary information10.1039/c4ra11704b

Cite this: RSC Adv., 2015, 5, 2285

Received 3rd October 2014Accepted 27th November 2014

DOI: 10.1039/c4ra11704b

www.rsc.org/advances

This journal is © The Royal Society of C

toluminescent carbon dots for thedetection of cobalt ions†

Chi-Lin Li,a Chih-Ching Huang,bcd Arun Prakash Periasamy,a Prathik Roy,a

Wei-Cheng Wu,ef Chia-Lun Hsua and Huan-Tsung Chang*a

We have developed a simple assay for the sensing of cobalt ions (Co2+), based on the analyte

induced photoluminescence (PL) quenching of carbon dots (C-dots). The C-dots (mean diameter

3.6 � 0.3 nm) prepared from L-cysteine through a simple hydrothermal process at 300 �C for 2 h

have a quantum yield of 13.2%. The C-dots have strong blue PL with a maximum PL intensity at

395 nm under an excitation wavelength of 325 nm. Through the reactions of Co2+ ions with cysteine

molecules/residues on the surfaces of the C-dots, non-photoluminescent CoxSy nanoparticles are

formed. As-formed CoxSy nanoparticles and the C-dots further form aggregates in the solution,

leading to PL quenching. The C-dot probe allows detection of Co2+ ions over a concentration

range from 10 nM to 100 mM (R2 ¼ 0.992). This reliable, rapid, sensitive, and selective C-dot probe

has been utilized for the determination of the concentrations of Co2+ ions in vitamin B12 and natural

water samples.

1 Introduction

Carbon dots (C-dots) are a fascinating class of newly synthesizednanomaterials for sensing, catalysis, fuel cells, and imaging.1–6

Low-cost C-dots with sizes <10 nm are promising photo-luminescent nanomaterials because of their interesting char-acteristics, including high dispersibility in aqueous solution,high photoluminescence (PL) quantum yield (ff), tunable PLexcitation and emission wavelengths, low photobleaching, andexcellent biocompatibility.1–6

The detection of cobalt (Co) has received considerableattention due to its signicant roles in chemical, biological, andenvironmental elds.7–9 In spite of the fact that Co is anessential element required in minute amounts as a componentof vitamin B12 for people (the recommended daily intake ofvitamin B12 is 6 mg), excessive Co uptake can lead to diseases

University, 1, Section 4, Roosevelt Road,

ntu.edu.tw; Fax: +886-2-33661171; Tel:

y, National Taiwan Ocean University, 2,

al Taiwan Ocean University, 2, Pei-Ning

aohsiung Medical University, Kaohsiung,

cience, National Tsing Hua University,

aiwan International Graduate Program,

(ESI) available: Fig. S1–S6. See DOI:

hemistry 2015

(e.g., allergic dermatitis, rhinitis, asthma, cardiomyopathy) anddeath.8 Exposure of rats to short-term high levels of cobalt in thedrinking water or food results in effects on the blood, liver,kidneys, and heart. The oral LD50 (lethal dose, 50%) value ofsoluble Co salts in rats is about 150 to 500 mg kg�1.10 Co is alsoconsidered as a possible carcinogen for humans and a toxicelement to aquatic organisms.8,9,11 Mean Co concentrations inrainwater are 0.3–1.7 mg mL�1, while that in soil vary widelyfrom 1 to 40 mg mL�1.12 Co concentrations in drinking waterare usually less than 1–2 mg mL�1.12 Themaximum level of Co indrinking water permitted should be lower than 40 mg L�1.12

Hence, it is important to develop sensitive, selective, and reli-able analytical techniques for the determination of theconcentrations of Co2+ in natural water samples and supple-ments (e.g. vitamin B12).

Many spectrophotometric techniques have been developedfor the detection of Co2+ ions, with advantages of high selec-tivity, sensitivity, and fast response.13–24 However, some of themare commercially unavailable, expensive, and hydrophobic.Herein, we report a label-free C-dot probe for the detection ofCo2+ ions in water samples, based on the analyte induced PLquenching (Scheme 1). The C-dots were prepared from cysteinethrough a hydrothermal process. Through specic interactionsbetween Co2+ and cysteine/residue on the surfaces of C-dots,CoxSy nanoparticles were formed. The as-formed CoxSy nano-particles and C-dots further formed aggregates in the solution.Practicality of this sensitive and selective C-dot probe wasdemonstrated by determination of the concentrations of Co2+

ions in stream and lake water and in vitamin B12 supplementsamples.

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Scheme 1 Schematic representation of hydrothermal preparation ofcarbon dots (C-dots) from cysteine for the detection of Co2+ ions.

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2 Experimental2.1 Chemicals

Cobalt sulfate, L-cysteine, ethylenediaminetetraacetate (EDTA),glycine, hydrochloric acid, nitric acid, quinine sulphate, and allof the metal salts used in this study were purchased fromSigma-Aldrich (Milwaukee, WI, USA). Monobasic and dibasicsodium phosphates obtained from J. T. Baker (Phillipsburg, NJ,USA) were used to prepare a phosphate buffer (50 mM, pH 9.0).Ultrapure water (18.2 MU cm) from a Milli-Q ultrapure watersystem (Millipore, Billerica, MA, USA) was used throughout theexperiments.

2.2 Synthesis of C-dots

Cysteine solution (1 M, 15 mL) prepared in ultrapure water washeated hydrothermally in a stainless steel autoclave at 300 �Cfor 2 h. The resulting brownish yellow solution was cooled toambient temperature (25 �C) and subsequently centrifuged at arelative centrifugal force (RCF) of 12 000g for 10 min to removelarge or agglomerated particles. The supernatant containing C-dots was then ltered through a 0.2 mm polyethersulfonemembrane to remove large particles. The as-puried solutioncontained C-dots at a concentration of 3.3 mg mL�1. Aliquots(14.6 mL) of the as-prepared solution containing C-dots werefreeze-dried in a lyophilizer for 24 h and stored at �20 �C indark. The puried C-dots were diluted to required concentra-tions with sodium phosphate buffer (10 mM, pH 9.0). The ff

value of C-dots was calculated by comparing their integrated PLintensity (excited at 365 nm) and absorbance at 365 nm withthose of quinine sulphate. Quinine sulphate (ff ¼ 0.54) wasdissolved in 0.1MH2SO4 (refractive index, 1.33) and C-dots weredispersed in water (refractive index, 1.33). The absorbancevalues of the two solutions in 1 cm (optical path length) cuvetteswere kept under 0.1 at their excitation wavelengths to minimizethe re-absorption effects. Excitation and emission slit widthswere both set at 5.0 nm.

2.3 Characterization of C-dots

Transmission electron microscopy (TEM) and high-resolutionTEM (HRTEM) images of C-dots were recorded using JEOLJSM-1230 (Hitachi, Tokyo, Japan) and FEI Tecnai-G2-F20(GCEMarket, NJ, USA) systems operating at 200 kV, respec-tively. Prior to conducting TEM measurements, the as-preparedC-dots (3.3 mg mL�1) were diluted 10-fold with ultrapure water.The as-diluted C-dots were carefully deposited on 400-mesh

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carbon-coated Cu grids and excess solvents were evaporated atambient temperature and pressure. A double-beam UV-Visspectrophotometer (Cintra 10e, Dandenong, VIC, Australia)was used to measure the absorption spectra of the C-dots inultrapure water. The PL spectra of the as-prepared C-dots wererecorded using a Cary Eclipse PL spectrophotometer (Varian,Palo Alto, CA, USA) that was operated at excitation wavelengthsin the range 325–405 nm. A Raman microscopic system with a50� objective (Dongwoo Optron, Kyunggi-do, Korea) was usedto analyze air-dried C-dots on a silicon wafer coupled with adiode laser at an excitation wavelength of 532 nm.

A Varian 640 FT-IR spectrophotometer was employed toanalyze possible functional groups present in the C-dots. Priorto X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbeXPS, Ulvac Technologies, Methuen, MA, USA) analysis, C-dotsolution (3.3 mg mL�1, 10 mL) was placed on a Si substrateand then dried at ambient temperature. The XPS spectrum of C-dots was recorded in a constant analyzer energy mode with apass energy of 28 eV and Al Ka (1486.6 eV) radiation as theexcitation source. A zetasizer (Nano-HT, Malvern, UK) wasemployed to record the dynamic light scattering (DLS) histo-gram and zeta potential of C-dots (0.33 mg mL�1). X-Raydiffraction (XRD) sample was prepared by depositing the C-dot solution (3.3 mg mL�1, 20 mL) on a silicon wafer. XRDmeasurement was performed by using a X-ray diffractometer (D/MAX 2200 VPC, Rigaku, Sendagaya, Shibuya-Ku, Tokyo, Japan)with the Cu Ka1 line (l ¼ 1.54 A, energy ¼ 8.8 keV).

2.4 Detection of Co2+ ions

Stock solutions of inorganic metal ions (0.1 M) were prepared in0.1 M HNO3, which were then diluted to required concentra-tions (0.1–1000 mM) in sodium phosphate buffer (10 mM, pH9.0). Aliquots (100 mL) of metal ion solutions were added sepa-rately to sodium phosphate buffer (10 mM, pH 9.0) containingC-dots (17 mg mL�1) to obtain nal volumes of 1 mL. Aerequilibrating at ambient temperature for 1 h, the mixtures weretransferred separately into 96-well microtiter plates and thentheir PL spectra were recorded using a Synergy 4 Multi-Modemonochromatic microplate spectrophotometer (Biotek Instru-ments, Winooski, VT, USA).

2.5 Analysis of Co2+ ions in real samples

Water samples collected from stream and lake near the campusof National Taiwan Ocean University were ltered through a 0.2mm membrane. For the detection of Co2+ ions, aliquots of thewater samples (100 mL) were spiked with standard Co2+ ionsolutions (100 mL, 0.1–5 mM). The spiked samples were thendiluted to 1 mL with sodium phosphate buffer (10 mM, pH 9.0)containing C-dots (17 mg mL�1). The nal concentrations ofCo2+ ions in the mixtures were over the range 10–500 nM.Vitamin B12 tablets were obtained from General NutritionCenters (GNC, Pittsburgh, PA, USA). The tablets were weighedand ground to ne powder. Vitamin B12 (1 mg) powder wasdissolved in 50 mL HCl solution (12 M) through ultrasonicationfor 2 h.25 The resulting solution was further diluted with 950 mLof ultrapure water. Aliquots of the diluted sample (100 mL) were

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Paper RSC Advances

spiked with standard Co2+ ions (0–200 mM; 100 mL) in sodiumphosphate buffer (10 mM, pH 9.0). The spiked samples werethen diluted to 1 mL with sodium phosphate buffer (10 mM, pH9.0) containing C-dots (17 mg mL�1). Aer incubation for 1 h,the mixtures were subjected to PL measurements with excita-tion and emission wavelengths of 325 and 395 nm, respectively.All the spiked natural water samples were also analyzed byinductively coupled plasma mass spectrometry (ICP-MS).

Fig. 2 (A) XRD spectrum, (B) FTIR spectrum, (C) C1s XPS spectrum, and(D) Raman spectrum of as-prepared C-dots. Other conditions werethe same as those described in Fig. 1.

3 Results and discussion3.1 Characterization of C-dots

The TEM image of as-prepared C-dots displayed in Fig. 1A(a)shows that they were uniform and monodispersed spheres witha mean diameter of 3.6 � 0.3 nm (from 100 counts). Thisautoclave-assisted hydrothermal carbonization method did notrequire any surface passivation agents or inorganic additives;the sole reagent was cysteine. The cysteine molecules wereconverted to C-dots via four steps, including dehydration,pyrolysis, carbonization, and passivation.26,27 During thecarbonization step, a short single burst of nucleation occurredin the solution as soon as a critical supersaturation level wasreached. The as-formed nuclei underwent isotropic growththrough the diffusion of solutes toward the particles surfacesand then converted to C-dots.28–31 The lattice structure(d-spacing, 0.32 nm) is discernible in the HRTEM image (theinset to Fig. 1A(a)), which is in good agreement with the XRDpattern (002 plane, at 2q ¼ 23.5�) as displayed in Fig. 2A.32

Fig. 1 (A) TEM images, (B) UV-Vis absorption spectra, (C) time-courselight scattering (excitation and emission wavelengths were both set at650 nm), and (D) PL emission spectra of as-prepared C-dots (17 mgmL�1) in sodium phosphate buffer (10 mM, pH 9.0) in the (a) absenceand (b) presence of 10 mM Co2+ ions, respectively. Inset to (A) (a)HRTEM image of C-dots. Inset to (B) photograph of C-dots solution inthe (a) absence and (b) presence of Co2+ ions. Excitation wavelengthfor PL measurement in (D) is 325 nm. Absorbance (Abs) in (B) andfluorescence intensities (IF) in (D) are plotted in arbitrary units (a. u.).Inset to (D) photograph of the C-dots solution in the (a) absence and(b) presence of 10 mM Co2+ ions excited by a UV lamp (365 nm).

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The functional groups in the C-dots were determined byFTIR (Fig. 2B). The peaks at the wavenumbers around 3300,3000, 1700 and 1100 cm�1 are assigned separately for thestretching vibrations of O–H/N–H, C–H, C]O, and C–O bonds,respectively. The peak at the wavenumber about 1400 cm�1 isdue to the CH3 band of bending vibration. The S–H stretchingvibration mode at the wavenumber close to 2600 cm�1 indicatesthat the C-dots have the thiol functional group on its surfaces.The presence of hydroxyl and carboxylate groups on the surfaceof C-dots was further conrmed with its negative zeta potentialvalue (�32.6� 3.7 mV; n¼ 5). The detailed C1s XPS spectrum ofC-dots (Fig. 2C) shows ve bands at the binding energies of284.5, 286.1, 287.4, 288.0 and 288.5 eV, which are attributed toC]C/C–C, C–O, C–S, C]O, and C–NHX bonds, respectively. TheRaman spectrum of the C-dots (Fig. 2D) displays D band and Gband at 1361 and 1542 cm�1, respectively. The D band is asso-ciated with vibrations of carbon atoms with dangling bonds inthe terminal plane of disordered carbon, while the G band isrelated to the vibration of sp2-hybridized carbon atoms in a two-dimensional hexagonal lattice.33 The two bands conrm thegraphitic domains present in the network of carbon atoms.

3.2 Sensing of Co2+ ions

Fig. 1A(b) reveals that as-prepared C-dots in the presence of10 mMCo2+ ions aggregated to form large granular C-dots/cobaltsulde (C-dots/CoxSy) nanomaterials with 40–80 nm in diam-eter. XPS results (Fig. S1A, ESI†) reveal that the C-dots/CoxSynanomaterials contain 56.1%, 20.6%, 9.6%, 8.8%, and 4.9% ofC, O, S, N and Co, respectively. The XRD patterns of the CoxSy(Fig. S1B, ESI†) show characteristic diffraction peaks at 2qvalues of 29.6� and 61.7�, which correspond to Co8S9 (111) andCoS2 (230), respectively. The representative STEM-EDS (Fig. S2,ESI†) of C-dots/CoxSy nanomaterials reveals that the C-dots andCoxSy coexisted and were homogeneously distributed. Co2+ ions

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Fig. 3 (A) PL emission spectra of C-dots (17 mg mL�1) upon addition ofCo2+ ions (0–100 mM) in sodium phosphate buffer (10 mM, pH 9.0). (B)Selectivity of the C-dots probe toward Co2+ ions against other metalions. The concentration of each of the metal ions was 10 mM. Inset to(A) (Upper) photograph of the C-dot solutions in the presence ofvarious concentrations of Co2+ (0–100 mM); (Down): a linear Log–Logplot was obtained over the concentration of Co2+ ions from 10 nM to100 mM. Error bars represent standard deviations from four repeatedexperiments.

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reacted with the cysteine/ligand on the surfaces of C-dots toform CoxSy nanoparticles. The as-formed CoxSy nanoparticlesand the unreacted C-dots formed aggregates in the solution.

The UV-Vis absorption spectrum (Fig. 1B) of the C-dotsexhibited two absorption bands at the wavelengths around

Table 1 Nanoparticles-based optical assays for the detection of Co2+ io

Method Probe unit LOD

Absorbance TGA functionalized CTAB-Au NPsa 30Absorbance P-Au NPsb 200Absorbance DDTC-Ag NPsc 14 00Absorbance Triazole-carboxyl Ag NPs 700Absorbance COF-CdS QDsd 39SERS TPY-DTC-Ag NPse

Chemiluminescence CTAB@carbon dotsf

Photoluminescence CePO4 : Tb3+ NCsg

Photoluminescence Mn-doped ZnS QDsh 6Photoluminescence TGA-CdTe QDsi

Photoluminescence CuInS2/ZnS/TGA QDsj 16Photoluminescence C-dots

a Thioglycolic acid (TGA) functionalized-hexadecyltrimethylammonium(CALNNDHHHHHH) modied Au NPs. c Dopamine dithiocarbamate (Ddots (QDs). e Dithiocarbamate anchored terpyridine (TPY-DTC) functionform the CTAB–PEG microenvironment. g CePO4 : Tb

3+ nanocrystals. h Ncapped CdTe QDs. j Thioglycolic acid-capped CuInS2/ZnS QDs.

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272 and 320 nm corresponding to p / p* (C]C bond) andn / p* (peroxide and/or epoxide groups) transitions, respec-tively.31,34 In the presence of Co2+ (10 mM), the color of solutionchanged from light yellow to gray black as a result of aggrega-tion of C-dots with the as-formed CoxSy (the inset in Fig. 1B).Fig. S3 (ESI†) illustrates the PL spectra of the C-dots, showingred shis of the PL with decreased intensities under excitationsat the wavelengths from 325 to 405 nm. These excitationwavelength-dependent PL properties are due to differenthybridized compositions, structures, emissive trap sites, andsizes of the C-dots.1–3 The C-dots have strong blue PL as shownin the inset (Fig. 1D), with a maximum PL intensity at 395 nmunder an excitation wavelength of 325 nm (Fig. S3, ESI†). The ff

value (lex ¼ 365 nm) was determined to be 13.2% when usingquinine sulfate (ff � 54% in 0.1 M H2SO4) as the reference. Theobtained ff value is comparable with that of C-dots preparedthrough hydrothermal methods.1–6,35,36 Co2+ induced PLquenching of the C-dots (Fig. 1D) as a result of the formation ofnon-photoluminescent CoxSy nanoparticles. PL quenching isalso possible through electron and energy transfer between theC-dots and CoxSy.1,37,38 The deposition of CoxSy on the surfacesof C-dots may interrupt photoinduced charge separation and/orradiative recombination of C-dots.37

Fig. 1C displays that the static scattering light intensities of theC-dots (17 mgmL�1) wasmuch higher than that in the presence of10 mM Co2+ ions, conrming the as-formed CoxSy nanoparticlesfurther formed aggregates with C-dots. To further understand theCo2+ induced PL quenching of C-dots, lifetimes of the C-dots inthe absence and presence of 10 mM Co2+ ions were measured.Fig. S4 (ESI†) shows that the lifetime of C-dots decreased from itsoriginal 6.26 to 2.51 ns, indicating that excited-state molecules ofC-dots reacted with Co2+ ions. In addition, the lifetime of C-dotsdecreased gradually upon increasing concentration of Co2+ ions(data not shown). The results revealed that Co2+ induced PLquenching is a dynamic quenching. To further prove that the PLquenching is related to the formation CoxSy nanoparticles andtheir aggregates with the C-dots, control experiments were done

ns

(nM) Real samples Ref.

0 Not provided 140 Tap/river water 150 Not provided 160 Drinking water 170 Drinking/tap/river/lake water 181 Potable water 190.67 HepG2 cells 203.5 Tap/river/lake water 210 Tap/pond water 227.3 Not provided 230 Simulated water 245 Lake/stream water and vitamin B12 tablet This study

bromide (CTAB) modied gold nanoparticles (Au NPs). b PeptideDTC) functionalized Ag NPs. d Carboxyl-functionalized CdS quantumalized Ag NPs. f PEG-passivated carbon dots modulated with CTAB to-Acetyl-L-cysteine-capped Mn-doped ZnS QDs. i Thioglycolic acid (TGA)-

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Paper RSC Advances

using C-dots that had been puried through dialysis. The PLintensity of the puried C-dots was slightly quenched by Co2+ ions(data not shown). In contrast, PL quenching of the puried C-dotsby Co2+ ions was signicant by adding cysteine (5 mM) to thesolution. The PL of C-dots quenched by Co2+ ions was minimizedby adding a strong metal ion-chelating agent such as EDTA to thesolution (Fig. S5, ESI†), revealing that the PL quenching associ-ated with CoxSy nanoparticles was minimized. In other words,the Co2+ ions induced PL quenching of C-dots was minimizedthrough a competitive complexation reaction.

Fig. 4 The linear relationship between [(IF � IF0)/IF0] and [Co2+] from10 to 500 nM in 10-fold diluted (A) lake and (B) streamwater samples insodium phosphate buffer (10 mM, pH 9.0). (C) Analysis of a repre-sentative vitamin B12 sample using C-dots probe in sodium phosphatebuffer (10 mM, pH 9.0). Aliquots of the diluted vitamin B12 sample(1000 mg mL�1) were spiked with Co2+ ions at concentrations in therange 0–20 mM. Other conditions were the same as those described inFig. 3.

3.3 Sensitivity and selectivity

Fig. 3A depicts the PL responses of the C-dots (17 mg mL�1)toward Co2+ ions (0–100 mM) in sodium phosphate buffer (10mM, pH 9.0). The value of logarithm of (IF0 − IF)/IF0 (IF and IF0are the PL intensities of the C-dots in the presence and absenceof Co2+ ions, respectively) of C-dots solution increased linearly(R2¼ 0.992) upon increasing the concentration of Co2+ ions overthe range from 10 nM to 100 mM. The C-dot probe alloweddetection of Co2+ ions as low as �5 nM (�0.3 mg mL�1), which ismuch below the maximum level of Co2+ ions permitted(�40 mg mL�1) for drinking water. Compared with othernanoparticles-based optical sensors,14–24 this approach opens upvarious avenues toward the detection of Co2+ ions in environ-mental and biological samples. We noted that only cysteinesolution without any hydrothermal treatment (control)allowed detection of Co2+ ions down to 1.0 mM through theabsorption measurement at 450 nm (Fig. S6, ESI†). Theabsorption band at 450 nm is due to the as-formed [Co(Cys)2]complexes. Relative to the cysteine-operated colorimetricapproach, the PL approach provides much better sensitivitytoward Co2+ ions. To investigate the selectivity of C-dots probetoward Co2+ ions, metal ions such as Na+, K+, Mg2+, Ca2+,Mn2+, Fe3+, Co2+, Cu2+, Zn2+, Hg2+, Pb2+, and Cd2+ ions (10 mM)were added separately into the probe solution. The relative PLquenching [(IF � IF0)/IF0] for other metal ions is presented inFig. 3B, where, IF and IF0 are the PL intensities at 395 nm of theC-dots in the presence and absence of metal ions, respectively.Fig. 3B reveals that the C-dot probe was specic toward Co2+

ions. The selectivity for Co2+ over the selected metal ionsresulted mainly from the specic interactions between Co2+

and cysteine.39,40 C-dots prepared from organic compounds(e.g., glycine, EDTA) did not exhibit any obvious selectivitytoward Co2+ ions (data not shown). In comparison with somerepresentative nanoparticles based optical methods (Table 1),our label-free PL-based assay for Co2+ ions is relatively simple,cost-effective, selective and sensitive. In addition, other tech-niques mostly require covalent conjugation of ligands tonanoparticles.14–24 As shown in Table 1, other kinds of QDshave been used for the detection of Co2+ ions. However, ourlabel-free PL-based assay provides much better sensitivitythan that of the reported ones, showing C-dots are the bestcandidate for the detection of Co2+ ions. Traditional toxicmetal-based semiconductor QDs have raised serious healthand environmental concerns. In addition, the composite andsize of the QDs (i.e., the bandgap of QDs) may inuence the

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selectivity and quenching efficiency of QDs toward heavymetal ions.41,42 Using toxic metal precursors and organicsolvents for the preparation of QDs are also problematic.Relative to semiconductor QDs, the preparation of C-dots issimple and straightforward. Avoiding use of toxic precursorsand organic solvents are advantageous. Recently, positioningQDs on top of TiO2 layer has been shown to be effective forimproving their PL.43 In our future work, C-dots will becoupled with different metal oxide nanostructures includingTiO2, ZnO or MnO2 for improving their PL and sensitivity ofthe detection of Co2+ ions.

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3.4 Detection of Co2+ ions in water and vitamin B12 samples

To test the practicality of our developed approach, a standardaddition method was applied to determine the concentration ofCo2+ ions in water and vitamin B12 samples. As shown in Fig. 4Aand B, the PL intensity ratio of C-dot probe decreased uponincreasing the spiked concentration of Co2+ ions (10–500 nM) inthe 10-fold diluted lake water and stream water samples,respectively. The limits of detection (LOD; S/N¼ 3) for Co2+ ions(�10 nM) in these two natural water samples were slightlyhigher than that obtained in the standard solutions, revealingsmall matrix effects. Neither the C-dots based sensing probe nordid an ICP-MS approach detect the presence of Co2+ ions in thenon-spiked water samples. Fig. 4C displays the PL responses ofthe C-dot probe toward Co2+ ions (0–20 mM) spiked into vitaminB12 samples. The concentration of Co2+ ions in the vitamin B12

tablet was determined to be 4.34 � 0.08 wt% (n ¼ 4). There wasno signicant difference (at a 95% condence level) between thevalue determined using our approach and the certied one (4.36wt%). The results reveal that the label-free C-dots probe can beutilized for the determination of the concentrations of Co2+ ionsin natural water and supplement samples.

4 Conclusions

C-dots were successfully synthesized from cysteine through ahydrothermal process. The as-synthesized C-dots were used forthe sensitive and selective detection of Co2+ ions in aqueoussolution, based on the analyte induced PL quenching. Thissimple, cost-effective, selective, and sensitive probe allows thedetection of Co2+ ions in vitamin B12 and natural water samples.This study revealed that the cysteine played an important role indetermining the selectivity. It is thus our belief that C-dotsprepared from various precursors are worthy to be tested fordifferent metal ions.

Acknowledgements

This study was supported by the Ministry of Science & Tech-nology (MOST) (contract NSC 98-2113M-002-011-MY3), theNational Taiwan Health Research Institutes Taiwan (contractNHRI-EX100-10047NI), and the Institute of Nuclear EnergyResearch (contract 1002001INER082). A.P.P and P.R are gratefulto the Department of Chemistry, National Taiwan Universityand MOST for postdoctoral fellowships under the contractnumbers 101-R-4000 and NSC 101-2113-M-002-002-MY3,respectively.

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