This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2019, 4, 1581−1591
http://pubs.acs.org/journal/acsodf
Antibacterial Nitrogen-doped Carbon Dots as a Reversible “Fluorescent Nanoswitch” and Fluorescent Ink Satyesh Raj Anand, Anshu Bhati, Deepika Saini, Gunture, Neetu Chauhan, Prateek Khare, and Sumit Kumar Sonkar*
ACS Omega 2019.4:1581-1591. Downloaded from pubs.acs.org by 193.56.67.200 on 01/18/19. For personal use only.
Department of Chemistry, Malaviya National Institute of Technology, Jaipur, Jaipur 302017, India ABSTRACT: The present finding describes an efficient facile approach for the fabrication of nitrogen-doped carbon dots (N-CDs) as a “fluorescent nanoswitch”. Highly fluorescent blue-light-emitting N-CDs have been synthesized via a simpler hydrothermal method using 2,2′-(ethylenedioxy)-bis(ethylamine) and malic acid as the precursors. N-CDs showed excitation-dependent and pH-independent emission along with a quantum yield of ∼25%. The blue fluorescent emission of N-CDs has been selectively “turned off” (quenching of fluorescence (FL)) during the sensing of Cr(VI) with 0.02 μM limit of detection and further been selectively “turned on” (restoration of FL) on sensing ascorbic acid, compared with other metal cations and biomolecules tested. For testing the practical applicability of N-CDs, the switchable reversibility of the fluorescent nanoswitch has been tested for up to four cycles on the basis of FL “on−off−on”. Furthermore, the toxicological test showed the antibacterial effect of the N-CDs on the tested Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli cells. Additionally, these N-CDs can also be used as a fluorescent ink for imaging purposes. Co(II)35 especially because of their FL emission properties, “turn-off” and “turn-on”. Cr(VI) is highly toxic in nature as compared to its lower valent state Cr(III), causing several adverse impacts on the environment as well as on the human health.36,37 Thus, their discharge into the environment through industrial and other anthropogenic activities needs to be monitored thoroughly. Therefore, it is essential to continuously observe the level of Cr(VI) in water media, which needs to be up to 0.9 μM, possibly via using simpler techniques with accuracy even at its lower level.29 In addition, the detection of ascorbic acid (AA) (generally known as vitamin C) also has been important for human health as AA is protecting us from many diseases. Although a few reports have already been documented for the detection of Cr(VI) and AA in the literature,38,39 the development of simpler and economic methodologies has always been in demand. For this, fluorescent properties of the CDs2,3,40 and doped CDs like N-CDs have been explored on the basis of their abilities of FL sensing. The present finding depicts a simpler methodology for the fabrication of N-CDs and their use as a reversible “fluorescent nanoswitch” for selective sensing of Cr(VI) and AA. N-CDs show excellent stability and solubility in aqueous medium, and their fluorescent properties do not change on addition of various other ions (interference study) and with the variation in the pH of the solution, which may offer the practical
1. INTRODUCTION Fluorescent carbon dots (CDs)1−6 are zero-dimensional carbon materials consisting of variable composition of sp2-/ sp3-hybridized carbon atoms with abundant functional groups over the shell, due to which they exhibit unique fluorescence (FL) properties.1 Since their discovery,2 CDs have triggered a great interest in a wide range of applications, from bioimaging,3,7 sensing,8,9 drug delivery,10,11 photocatalysis,12 energy storage,13 and fluorescent ink14,15 to disease diagnosis,16,17 and also in the field of agriculture18 because of their superior unique properties such as the biocompatibility, robust chemical inertness, nontoxicity,19 and stability.5,20−22 However, to accomplish more, presently the long-known method of doping has been articulating the optical properties of CDs as in the form of emergent doped-CDs.7,23,24 The doping of heteroatoms significantly affects the physicochemical and photochemical properties of CDs.7,24 Of all of the dopant materials, the doping of N has been a bit more feasible concerning its easier insertion in the carbonaceous matrix of CDs because of its comparable size to that of carbon along with the extra valence electron. Additionally, the emphasis on escaping the complicated synthesis methodologies also has been in demand, such as the exploration of newer sustainable approaches based on the hydrothermal,25 solvothermal26,27 and microwave-assisted2,3,28 methodologies for the facile fabrication of doped CDs as these are easier and scalable methodologies with an extra advantage of ease in reproducibility. From the many potential applications, currently, N-CDs are being used for sensing the heavy metal ions like Cr(VI),29 Hg(II),9 Fe(III),30 Cu(II),31 Zn(II),32 Cd(II),33,34 and © 2019 American Chemical Society
Received: November 15, 2018 Accepted: January 2, 2019 Published: January 17, 2019 1581
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Scheme 1. Schematic Representation Showing the Synthesis and Multifunctional Application of N-CDs
Figure 1. (a) TEM micrograph of N-CDs; (b) particle size distribution histogram of N-CDs; (c) HRTEM image of N-CDs, and graphitic fringes are marked with yellow circles; (d) HRTEM image showing a graphitic arrangement in a single N-CD.
application of N-CDs in the field of sensing as a reversible fluorescent nanoswitch. Also, these N-CDs worked as an antibacterial agent for inhibiting the growth of the tested Gram-positive S. aureus and Gram-negative E. coli cells in the
aqueous phase. Additionally, these were used as a blue-emitting fluorescent ink for imaging purposes. 1582
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Figure 2. (a) UV−vis absorption spectrum of N-CDs (the inset shows the photographs comparing the aqueous solution of N-CDs under day light (left) and UV light (right)); (b) FL emission spectra of N-CDs at varying excitations from 320 to 450 nm (the inset shows the stability of FL for 2 h under the continuous irradiation at 320 nm); (c) photoluminescence excitation spectra recorded at 445 nm emission; and (d) FL intensity of NCDs in different pH solutions.
Figure 3. (a) FT-IR and (b) survey XPS of N-CDs; (c) element composition and their corresponding high-resolution scans for: (d) C1s, (e) N1s, and (f) O1s.
1583
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Figure 4. (a) Photographic image of the aqueous solution of N-CDs with different metal ions under the UV-light illumination showing the selective sensing of Cr(VI) (turn-off); (b) FL spectra of N-CDs on addition of different metal ions (λex: 320 nm); (c) FL intensity ratio (Io − I)/Io of N-CD solution with different metal ions; (d) photographic image showing the selectivity of AA toward regaining the FL intensity of the maximally quenched solution of the N-CD−Cr(VI) complex with different biomolecules; (e) FL spectra of the N-CD−Cr(VI) complex showing the regain in the FL intensity on selective addition of AA (λex: 320 nm) with different metal ions under UV-light illumination; (f) FL intensity ratio (Io − I)/Io of the N-CD−Cr(VI) complex with different biomolecules.
2. RESULTS AND DISCUSSION
emitting N-CDs show excellent solubility with excitationdependent emission having quantum yield (QY) ∼ 25%. These N-CDs work as a reversible nanoswitch up to four cycles for the sensing of Cr(VI) as “FL turn-off” and AA as “FL turn-on”. The N-CDs also show antibacterial activity and work as a blue fluorescent ink.
A simpler hydrothermal technique has been used for the synthesis of N-CDs by mixing 2,2′-(ethylenedioxy)-bis(ethylamine) and DL-malic acid. The schematic representation depicting the hydrothermal synthesis and diverse applications of N-CDs is shown in Scheme 1. The as-synthesized blue-light1584
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
2.1. Microscopic and Spectroscopic Analyses. The morphological and microstructural analyses of N-CDs were performed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The TEM image of N-CDs in Figure 1a shows the uniform distribution of N-CDs, along with their size distribution histogram, Figure 1b, with Gaussian fitting, which confirms that N-CDs own a size distribution between 2 and 7 nm with an average diameter of ∼4.4 ± 0.87 nm. More structural information from Figure 1c confirms the quasi-spherical nature of N-CDs (encircled by yellow circles). The lattice spacing of N-CDs imaged by HRTEM (Figure 1d) is 0.32 nm, which highlights the crystalline nature of N-CDs. The optical properties of N-CDs (Figure 2) were characterized by UV−vis and FL spectroscopy techniques. N-CDs exhibit strong absorbance in the UV region, which further extended throughout the visible region. The absorption spectrum in Figure 2a displays three characteristic peaks: first at ∼284 nm corresponding to π−π* transitions of the aromatic sp2 domains and rest two peaks at ∼320 nm (a small hump) and at ∼668 nm can be attributed to n−π* transitions due to trapping of excited-state energy by the surface states, which is a characteristic property of the graphene-like structures.41 The N-CDs show a yellowish-brown color (Figure 2a; inset left) under normal day light, which turns blue under the illumination of UV light (Figure 2a; inset right). The emissive properties of N-CDs were further studied at different excitation wavelengths varying from 320 to 450 nm, as shown in Figure 2b. N-CDs feature excitation-wavelength-dependent emission in the range of 420−520 nm, which is attributed to particle size distribution with various lateral dimensions or emissive states owing to different surface states, traps, and edge structures.42−45 The N-CDs show good photostability up to 2 h with negligible bleaching under the continuous excitation of 320 nm, as shown in the inset of Figure 2b. The highest FL emission intensity was observed at 445 nm (under excitation of 320 nm). The photoluminescence excitation study of N-CDs shows three excitation peaks: at ∼389 nm (3.19 eV), ∼361 nm (3.43 eV), and ∼252 nm (4.92 eV), as displayed in Figure 2c, supporting the various emitting centers over the surface of NCDs. Along with this, the influence of the change in the pH on the FL properties of N-CDs was investigated (Figure 2d, which shows almost negligible effect of the pH on the FL intensity of N-CDs). N-Doping and the surface functionalization of the N-CDs were further confirmed by Fourier transform infrared (FT-IR) analysis. Figure 3a displays the FT-IR spectrum of N-CDs showing the two significant peaks merged with small weak vibrations located in the 3400−2800 cm−1 region. The broad and merged peaks at around ∼3497 and ∼3295 cm−1 correspond to the −NH and −OH stretching vibrations, respectively. A weak peak at ∼3080 cm−1 corresponds to the (C−H) stretching vibration. The medium peaks at around ∼2920 and 2869 cm−1 arise due to the presence of (−C−H) stretching vibrations. A strong peak at around ∼1644 cm−1 is attributed to the presence of (CO/CN) stretching vibrations46 of the amide group, whereas the peak at 1532 cm−1 is due to (OC−NH/CC) stretching vibrations.47 The weak peaks at ∼1454 and ∼1350 cm−1 correpond to the (−C−N) stretching vibration and −C−NH stretching vibration,48 respectively. The peak at ∼1287 cm−1 corresponds to the epoxy group (C−O−C). A strong and sharp peak at ∼1090 cm−1 is attributed to the alkoxy (−C−O) stretching vibration.49 The X-ray photoelectron spectroscopy (XPS)
analysis was carried out to reveal the extent of N-doping and for characterization of different binding species of carbon, nitrogen, and oxygen in N-CDs. The XPS survey spectrum of N-CDs, Figure 3b, shows peaks at ∼285.6, ∼400.5, and ∼533.0 eV corresponding to C1s (79.9%), N1s (8.1%), and O1s (12.0%), respectively with their element composition as shown in Figure 3c. Figure 3d shows the high-resolution C1s spectrum, which can be deconvoluted into five peaks at 284.1 eV (CC), 284.6 eV (C−C), 285.5 Ev (C−N), 286.4 eV (C−O), and 288.0 eV (CO). Figure 3e shows the highresolution N1s spectrum, which depicts peaks at 399.3, 400.9, and 401.2 eV associated with C−N−C, −N(C)3, and −N−H binding, respectively. In the high resolution spectrum of O1s, Figure 3f, three main components correspond to the C−O, CO, and O−CO bindings located at 531.9, 532.8, and 534.1 eV, respectively. N-CDs exhibit negatively charged surface functionalized groups supported by the FT-IR and XPS analyses, which generate localized energy levels42 with different emission centers. These emissive centers prefer nonradiative recombination of charge carriers, present in the intrinsic states with the utmost defects responsible for the emission which leads to the characteristic excitation-dependent FL emission. 2.2. N-CDs as Fluorescent Nanoswitch for Sensing of Cr(VI) and AA. The present work describes the newer perspective of N-CDs as a FL-based reversible fluorescent nanoswitch. For the fabrication of the reversible fluorescent nanoswitch, the idea requires the initial turn-off in the FL emission of the N-CDs via the addition of some quencher and further regaining of the FL of the quenched solution after the addition of some other specific molecule. For the same, herein, the FL emission of N-CDs was selectively quenched (turn-off) to its maximum via the addition of Cr(VI), followed by regaining of FL emission of the quenched solution (turn-on) via the addition of AA. N-CDs selectively sensed Cr(VI) and showed strong quenching in their FL emission in comparison to that from many other metal ions tested, such as Bi(III), Co(II), Cu(II), Ni(II), Sr(III), Zn(II), Hg(II), and Cr(III) ions. The selective, specific quenching affinity of Cr(VI) was observed from the photographic images of the respective solutions in the UV-light illumination, as shown in Figure 4a. The selective FL quenching of N-CDs was observed in the case of Cr(VI), whereas the other metal ions did not show quenching. In addition to photographic images, the selective FL quenching of N-CDs by Cr(VI) was also demonstrated by FL spectroscopy, as shown in Figure 4b. A qualitative change in the FL intensity for the peak centered at ∼445 nm of N-CDs decreased specifically with Cr(VI), compared with the other metal ions. The selective sensing of Cr(VI) has been displayed using (Io − I)/Io, where Io and I are the intensities of N-CDs without and with metal ions, respectively, as shown in Figure 4c. However, the novelty, being presented here, was the restoration of turn-on of the quenched emission of N-CDs via the selective addition of AA. The basic idea was simple, that is, just to release the loosely bound Cr(VI) from the surface of the N-CDs, which would result in the regaining of the FL intensity. Figure 4d shows the UV-illuminated photographic images of the respective solution showing the specific selectivity of AA toward the turn-on of the FL from the maximally quenched solution of the N-CD−Cr(VI) complex. From the many other common interfering biomolecules tested such as adenine (Ad), L-cyteine (Cy), dextrose (Dt), L-ascorbic acid (AA), guanine (Gu), L-proline (Pr), sulphonilic acid (SA), 1585
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Figure 5. (a) Schematic illustration showing the plausible mechanism for the cycle of N-CDs as a reversible fluorescent nanoswitch for “on−off− on” and (b) photographic image under UV-illumination for the four cycles, supporting the concept of N-CDs as a reversible fluorescent nanoswitch.
thymine (Th), and glycine (Gl), the specific selectivity of AA toward Cr(VI) can easily be explained by the long-known favorable bonding interactions between AA and Cr(VI) because of the higher affinity of AA toward Cr(VI) compared with N-CDs.50,51 The quenched FL of the N-CD−Cr(VI) complex turned on after subsequent addition of AA, Figure 4e. The same, selective turn-on in the FL intensity of the quenched solution of the N-CD−Cr(VI) complex using (Io − I)/Io with different biomolecules has been displayed in Figure 4f. Moreover, the reusability of this reversible fluorescent nanoswitch concerning its cyclability toward the turn-off and turn-on while selectively sensing Cr(VI) and AA was checked and is demonstrated in Figure 5 up to four cycles. Figure 5a demonstrates the possible mechanism for the working of the reversible fluorescent nanoswitch on the basis of the longknown DPC assay,3,52−54 selectively used for the detection of Cr(VI). The quenching of the N-CDs FL can be explained by ion-selective nature of surface functional groups of N-CDs and superior charge/energy transfer effects.41 Based on the DPC assay, while interacting with AA, almost all Cr(VI) gets converted into Cr(III) and forms an AA−Cr complex.44,55 The reversibility of the fluorescent nanoswitch can be easily visualized under UV-light illumination, as displayed in Figure 5b. The interaction between N-CDs and Cr(VI) leads to a loosely bound complex, that is, the N-CD−Cr(VI) complex, that results in turn-off of its visible FL. Furthermore, this loosely bound Cr(VI) may further have been attracted toward AA because of the well-known strong affinity toward Cr(VI).56−58
As the negative consequences of the presence of toxic Cr(VI) in water as a contaminant are well known, its active sensing followed by its removal from water is important. Owing to this, a FL-based study was carried out showing the effect of varying concentrations of Cr(VI), in the range of 0.02−400 μM, on the FL intensity of N-CDs toward a quantitative determination of Cr(VI). On gradually increasing the concentration of Cr(VI), N-CD emission showed a linear relation with quenching, as shown in Figure 6a. The zoomed image in Figure 6b (of the area marked with a red rectangular box) clearly shows a linear trend. The FL-quenching effect of N-CDs can be quantitatively determined by the following Stern−Volmer equation. Io/I = 1 + KSV[Cr(VI)]
where Io is the FL intensity of N-CDs without Cr(VI), I is the FL intensity in the presence of Cr(VI), and KSV is the static Stern−Volmer constant. Figure 6c shows a plot of Io/I versus increasing conc. of Cr(VI) (0.02−90 μM) for the Cr(VI)−NCDs system, which shows a linear relation with the regression coefficient (R2). The KSV value of ∼0.0018 × 106 L mol−1 is obtained from the slope of a linear fitted curve with a good correlation coefficient (R2) of 0.9936. The limit of detection was found to be ∼0.02 μM for Cr(VI), as calculated by taking the FL intensity to be equal to ∼3 times the standard deviation of the intensity (blank) divided by KSV. The synthesized NCDs can significantly determine the lower permissible limits of Cr(VI), ∼0.9 μM, in drinking water as defined by the World Health Organization.29 The same trend with the turn-on in the FL intensity via the selective addition of AA has been shown in 1586
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Figure 6. (a) FL quenching spectra of N-CDs by the addition of Cr(VI) at varying concentrations (0−400 μM); (b) zoomed FL spectra of (a); (c) Stern−Volmer plot of the quenching of N-CDs emission intensity centered at 445 nm by Cr(VI); (d) FL spectra after the addition of AA in the NCD−Cr(VI) complex (with concentration range 0−350 μM) in the N-CDs/Cr(VI)/AA system (FL turn-on); (e) zoomed FL spectra of (d) (0−120 μM) and (f) Stern−Volmer plot of the FL turn-on of N-CDs with a linear fitting by the continuous addition of AA.
Figure 7. (a) Effect on the FL intensity in the presence of foreign metal ions with coexisting Cr(VI) and (b) corresponding tolerance limits for sensing 5 mM Cr(VI) using N-CDs.
show any apparent changes in the selectivity for Cr(VI), even in the presence of several coexisting ions. The tolerance limits with different coexisting metal ions are presented in a tabular form Figure 7b. The highly selective sensing of Cr(VI) up to 5 mM was revealed with negligible interference from foreign metal ions (∼2-fold higher than that of Cr(VI)). 2.4. Antibacterial Effects of N-CDs Exposure to the E. coli and S. aureus Bacterial Cells. The as-synthesized NCDs showed a strong antibacterial effect on the two different strains of bacterial cells tested, that is, E. coli and S. aureus. The antibacterial activity was tested by optical density (OD) methods, as shown in Figure 8a,c, for E. coli and S. aureus, respectively. The optical density of both types of bacteria decreased on increasing the dose amount beyond 25 μL of N-
Figure 6d. Figure 6e shows the zoomed image (of the area marked with a rectangular box) of the FL intensities with the stepwise addition of AA showing the turn-on in the FL. A linear relationship has been shown in Figure 6f for the increasing concentration of AA verses Io/I of the FL intensity, which turned on for the maximally quenched solution of the N-CD−Cr(VI) complex. The limit of detection for AA was observed to be ∼0.01 μM, as calculated from the slope of the linear fitted curve (Figure 6f) with a good correlation coefficient (R2) of 0.981. 2.3. Tolerance Study for Sensing Cr(VI). Toward the practical applications of N-CDs as a reversible fluorescent nanoswitch, their tolerance limit under the influence of several coexisting metal ions has been analyzed. Figure 7a does not 1587
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Figure 8. Optical density (OD)-based growth bars at 600 nm (OD600) with the standard error showing the antibacterial effect of N-CDs at different concentrations tested on (a) E. coli and (c) S. aureus cells. The same was analyzed based on the well-plate experiment for (b) E. coli and (d) S. aureus.
Figure 9. Photographic image of paper-cut graphic of different animal stamps using N-CDs as a fluorescent ink under (a) day light and (b) UV light illumination.
3. CONCLUSIONS
CDs per mL of bacterial media. In the case of E. coli, 100% antibacterial activity was observed when the concentration was 25 μL of N-CDs per mL of media, whereas in the case of S. aureus, a little strong strength of N-CDs was required but its growth suppressed sharply at a higher amount of N-CDs (75 μL mL−1). The difference in antibacterial activity can be attributed to the difference in the outer structure of E. coli, Gram-negative bacteria, and S. aureus, Gram-positive bacteria, and needs to be investigated in detail. 2.5. N-CDs as Fluorescent Ink. N-CDs can be used as a fluorescent ink because of their excellent FL properties as nowadays the fluorescent ink is being popularized for printing purposes and in future can be used for the purpose of information storage and encryption. Figure 9 shows the pictorial image of different animal stamps using the N-CDs imprinted on the butter paper under day light (as control (Figure 9a)) and UV light irradiation (Figure 9b).
N-CDs were used for the selective and sensitive FL sensing of Cr(VI) and AA based on the simpler FL turn-off/turn-on mechanism in aqueous medium. The sensing ability of N-CDs as a reversible fluorescent nanoswitch allows comfortable naked-eye-based detection, with high selectivity−sensitivity, tolerance, and cycling stability under simpler operational conditions. The antibacterial activity of N-CDs can be explored in detail for the development of an antibacterial nanoprobe for successful inhibition of bacterial growth and contamination. Because of the excellent FL stability of the N-CDs, they have the potential to be used as a fluorescent ink. This strategy, together with the simplicity, can further expand the application of N-CDs in the analytical field as a fluorescent nanoswitch for sensing other systems along with inhibiting the growth of bacterial cells. 1588
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
4. EXPERIMENTAL SECTION 4.1. Materials. All metal ions used for the selectivity test were of nitrate salts, except for potassium dichromate (K2Cr2O7), mercuric chloride (HgCl2), barium chloride (BaCl2), calcium chloride (CaCl2), and all were purchased from Sigma-Aldrich, India. Adenine (Ad), L-cyteine (Cy), dextrose (Dt), L-ascorbic acid (AA), guanine (Gu), L-proline (Pr), sulphonilic acid (SA), thymine (Th), and glycine (Gl) were also purchased from Sigma-Aldrich, India. All of the reagents were of analytical grade and used as received. The deionized (DI) water was used throughout the synthesis and sensing experiments unless otherwise stated. 4.2. Synthesis of N-CDs. N-CDs were synthesized through a hydrothermal reaction using 2,2′-(ethylenedioxy)bis(ethylamine) as the N source and DL-malic acid as the carbon precursor. Malic acid (1.5 g) was added in ∼3 mL of 2,2′-(ethylenedioxy)-bis(ethylamine), and then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. After keeping the mixture at 150 °C for 6 h and cooling down to room temperature naturally, a red-brown solution was obtained. The N-CDs were collected via centrifugation at 8000 rpm for 15 min. The QY was measured by the following formula by taking the reference standard dye quinine sulfate (QY ∼ 54%).
solution (5 mM) was added to the N-CD−Cr(VI) mixture in progressively 21 steps. In all of the experiments, final volumes were maintained to 2 mL. The interference effect of other biomolecules, such as Ad, Cy, Dt, Gu, Pr, SA, Th, and Gl, was analyzed individually under the same experimental conditions. All of the FL spectral measurements were performed at fixed excitation and emission slits. 4.5. Antibacterial Test. E. coli and S. aureus cells were grown separately in Lysogeny broth media. The antibacterial assay for N-CDs against different bacterial strains was performed in 96-well plates. E. coli and S. aureus were incubated separately with varying concentrations of sterilized N-CDs (10, 25, 50, and 75 μL mL−1 of media) at 37 °C for 12 h in a dark incubator shaker. The optical densities of the samples with respect to the control were measured at 600 nm excitation.
■
Corresponding Author
*E-mail:
[email protected]. ORCID
Gunture: 0000-0001-5293-9173 Sumit Kumar Sonkar: 0000-0002-2560-835X Notes
The authors declare no competing financial interest.
2
Q = Qr ×
AUTHOR INFORMATION
A I ∩ × r × 2 Ir A ∩r
■
ACKNOWLEDGMENTS S.R.A. thanks DST, New Delhi, for funding; A.B. thanks MNIT Jaipur for a doctoral fellowship. D.S. thanks DST Inspire doctoral fellowship. G. thanks CSIR for senior research fellowship, and P.K. thanks CSIR (project no. 01(2854)/16/ EMR-II) and S.K.S. thanks DST (SB/EMEQ-383/2014) and CSIR (01(2854)/16/EMR-II) for funding. S.K.S. thanks Material Research Centre (MRC), MNIT Jaipur, for material characterizations. S.K.S. thanks to Sankalp Verma and Dr. Vivek Verma from Indian Institute of Technology, Kanpur for the bacterial experiment.
Here, Q is the quantum yield, I is the integrated emission intensity, ∩ represents the solvent refractive index (1.33 for water and 1.34 for 0.5 M H2SO4), A is the optical density, and the subscript “r” indicates the reference dye. Quninine sulfate was dissolved in 0.5 M H2SO4, whereas N-CDs were dissolved in water. 4.3. Instrumentation. The morphological characterizations were performed with TEM and HRTEM using a Tecnai 20 G2 electronic microscope with an accelerating voltage of 300 kV. Samples for TEM/HRTEM were prepared by placing a small drop of aqueous suspension of N-CDs onto a carboncoated copper grid and following solvent evaporation in a vacuum oven. The UV−vis absorption spectra were obtained at room temperature with a PerkinElmer Lambda 35 spectrometer. Photoluminescence spectral analysis was performed in aqueous solutions at room temperature with a PerkinElmer LS55 spectrophotometer. FT-IR spectra were obtained using pressed KBr pellets with a Bruker Vertex 70 FT-IR spectrophotometer. XPS measurements were carried out using ESCA+ omicron nanotechnology, Oxford Instruments. 4.4. FL Detection of Cr(VI) and AA. The detection of Cr(VI) and AA using N-CDs was performed in DI water at room temperature. The FL intensity of N-CDs was analyzed at 320 nm excitation wavelength and denoted Io (FL intensity at the initial point). Following that, 400 μL of Cr(VI) solution (5 mM) was added to the N-CD solution (2 mL) progressively in 26 steps. The final volume of mixtures was maintained to 2 mL at room temperature, and FL spectra were obtained after 2 min of incubation. The selectivity of Cr(VI) was assessed in triplicate under the same experimental conditions on the basis of variation in FL intensity using Cr(VI), Bi(II), Cu(II), Ni(II), Co(II), Sr(III), Cr(III), Zn(II), and Hg(II). The detection of AA was assessed by the FL recovery of the N-CD solution, quenched with Cr(VI). Typically, 0−350 μL of AA
■
REFERENCES
(1) Wang, Y.; Zhu, Y.; Yu, S.; Jiang, C. Fluorescent carbon dots: rational synthesis, tunable optical properties and analytical applications. RSC Adv. 2017, 7, 40973−40989. (2) Bhati, A.; Anand, S. R.; Gunture; Garg, A. K.; Khare, P.; Sonkar, S. K. Sunlight-Induced Photocatalytic Degradation of Pollutant Dye by Highly Fluorescent Red-Emitting Mg-N-Embedded Carbon Dots. ACS Sustainable Chem. Eng. 2018, 6, 9246−9256. (3) Khare, P.; Bhati, A.; Anand, S. R.; Gunture; Sonkar, S. K. Brightly Fluorescent Zinc-Doped Red-Emitting Carbon Dots for the Sunlight-Induced Photoreduction of Cr(VI) to Cr(III). ACS Omega 2018, 3, 5187−5194. (4) Babar, D. G.; Sonkar, S. K.; Tripathi, K. M.; Sarkar, S. P2O5 Assisted Green Synthesis of Multicolor Fluorescent Water Soluble Carbon Dots. J. Nanosci. Nanotechnol. 2014, 14, 2334−2342. (5) Tripathi, K. M.; Sonker, A. K.; Sonkar, S. K.; Sarkar, S. Pollutant Soot Of Diesel Engine Exhaust Transformed To Carbon Dots For Multicoloured Imaging Of E. coli And Sensing Cholesterol. RSC Adv. 2014, 4, 30100−30107. (6) Liao, J.; Cheng, Z.; Zhou, L. Nitrogen-Doping Enhanced Fluorescent Carbon Dots: Green Synthesis and Their Applications for Bioimaging and Label-Free Detection of Au3+ Ions. ACS Sustainable Chem. Eng. 2016, 4, 3053−3061. (7) Sun, Y.-P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang, H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S.-Y. Quantum-
1589
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
Sized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756−7757. (8) Gao, W.; Ma, Y.; Zhou, Y.; Song, H.; Li, L.; Liu, S.; Liu, X.; Gao, B.; Liu, C.; Zhang, K. High photoluminescent nitrogen-doped carbon dots with unique double wavelength fluorescence emission for cell imaging. Mater. Lett. 2018, 216, 84−87. (9) Patir, K.; Gogoi, S. K. Facile Synthesis of Photoluminescent Graphitic Carbon Nitride Quantum Dots for Hg2+ Detection and Room Temperature Phosphorescence. ACS Sustainable Chem. Eng. 2018, 6, 1732−1743. (10) Singh, R. K.; Patel, K. D.; Mahapatra, C.; Kang, M. S.; Kim, H.W. C-Dot Generated Bioactive Organosilica Nanospheres in Theranostics: Multicolor Luminescent and Photothermal Properties Combined with Drug Delivery Capacity. ACS Appl. Mater. Interfaces 2016, 8, 24433−24444. (11) Chakraborty, D.; Sarkar, S.; Das, P. K. Blood Dots: Hemoglobin-Derived Carbon Dots as Hydrogen Peroxide Sensors and Pro-Drug Activators. ACS Sustainable Chem. Eng. 2018, 6, 4661− 4670. (12) Bhattacharyya, S.; Ehrat, F.; Urban, P.; Teves, R.; Wyrwich, R.; Döblinger, M.; Feldmann, J.; Urban, A. S.; Stolarczyk, J. K. Effect of nitrogen atom positioning on the trade-off between emissive and photocatalytic properties of carbon dots. Nat. Commun. 2017, 8, No. 1401. (13) Devadas, B.; Imae, T. Effect of Carbon Dots on Conducting Polymers for Energy Storage Applications. ACS Sustainable Chem. Eng. 2018, 6, 127−134. (14) Ran, X.; Qu, Q.; Li, L.; Zuo, L.; Zhang, S.; Gui, J.; Kang, Y.; Yang, L. One-Step Synthesis of Novel Photoluminescent NitrogenRich Carbon Nanodots from Allylamine for Highly Sensitive and Selective Fluorescence Detection of Trinitrophenol and Fluorescent Ink. ACS Sustainable Chem. Eng. 2018, 11716. (15) Bhati, A.; Anand, S. R.; Saini, D.; Khare, P.; Dubey, P.; Sonkar, S. K. Self-doped nontoxic red-emitting Mg−N-embedded carbon dots for imaging, Cu(ii) sensing and fluorescent ink. New J. Chem. 2018, 42, 19548−19556. (16) Zheng, M.; Ruan, S.; Liu, S.; Sun, T.; Qu, D.; Zhao, H.; Xie, Z.; Gao, H.; Jing, X.; Sun, Z. Self-Targeting Fluorescent Carbon Dots for Diagnosis of Brain Cancer Cells. ACS Nano 2015, 9, 11455−11461. (17) Zhang, Y.; Wu, M.; Wu, M.; Zhu, J.; Zhang, X. Multifunctional Carbon-Based Nanomaterials: Applications in Biomolecular Imaging and Therapy. ACS Omega 2018, 3, 9126−9145. (18) Bhati, A.; Gunture; Tripathi, K. M.; Singh, A.; Sarkar, S.; Sonkar, S. K. Exploration of nano carbons in relevance to plant systems. New J. Chem. 2018, 42, 16411−16427. (19) Yang, S.-T.; Wang, X.; Wang, H.; Lu, F.; Luo, P. G.; Cao, L.; Meziani, M. J.; Liu, J.-H.; Liu, Y.; Chen, M.; Huang, Y.; Sun, Y.-P. Carbon Dots as Nontoxic and High-Performance Fluorescence Imaging Agents. J. Phys. Chem. C 2009, 113, 18110−18114. (20) Tripathi, K. M.; Bhati, A.; Singh, A.; Sonker, A. K.; Sarkar, S.; Sonkar, S. K. Sustainable Changes In The Contents Of Metallic Micronutrients In First Generation Gram Seeds Imposed By Carbon Nano-Onions: A Life Cycle Seed To Seed Study. ACS Sustainable Chem. Eng. 2017, 5, 2906−2916. (21) Dubey, P.; Tripathi, K. M.; Mishra, R.; Bhati, A.; Singh, A.; Sonkar, S. K. A Simple One-Step Hydrothermal Route Towards Water Solubilization Of Carbon Quantum Dots From Soya-Nuggets For Imaging Applications. RSC Adv. 2015, 5, 87528−87534. (22) Wang, R.; Lu, K.-Q.; Tang, Z.-R.; Xu, Y.-J. Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis. J. Mater. Chem. A 2017, 5, 3717−3734. (23) Tripathi, K. M.; Bhati, A.; Singh, A.; Gupta, N. R.; Verma, S.; Sarkar, S.; Sonkar, S. K. From The Traditional Way Of Pyrolysis To Tunable Photoluminescent Water Soluble Carbon Nano-Onions For Cells Imaging And Selective Sensing Of Glucose. RSC Adv. 2016, 6, 37319−37340. (24) Luo, P. G.; Yang, F.; Yang, S.-T.; Sonkar, S. K.; Yang, L.; Broglie, J. J.; Liu, Y.; Sun, Y.-P. Carbon-Based Quantum Dots For
Fluorescence Imaging Of Cells And Tissues. RSC Adv. 2014, 4, 10791−10807. (25) Li, H.; Yan, X.; Qiao, S.; Lu, G.; Su, X. Yellow-Emissive Carbon Dot-Based Optical Sensing Platforms: Cell Imaging and Analytical Applications for Biocatalytic Reactions. ACS Appl. Mater. Interfaces 2018, 10, 7737−7744. (26) Zhao, J.; Huang, M.; Zhang, L.; Zou, M.; Chen, D.; Huang, Y.; Zhao, S. Unique Approach To Develop Carbon Dot-Based Nanohybrid Near-Infrared Ratiometric Fluorescent Sensor for the Detection of Mercury Ions. Anal. Chem. 2017, 89, 8044−8049. (27) Grocholl, L.; Wang, J.; Gillan, E. G. Solvothermal Azide Decomposition Route to GaN Nanoparticles, Nanorods, and Faceted Crystallites. Chem. Mater. 2001, 13, 4290−4296. (28) Wang, H.; Li, D.; Liu, J.; Liu, L.; Zhou, X.; Qu, Y.; Zhang, J.; Feng, Y. Microwave-assisted synthesis of nitrogen-doped activated carbon as an oxygen reduction catalyst in microbial fuel cells. RSC Adv. 2016, 6, 90410−90416. (29) Rong, M.; Lin, L.; Song, X.; Wang, Y.; Zhong, Y.; Yan, J.; Feng, Y.; Zeng, X.; Chen, X. Fluorescence sensing of chromium (VI) and ascorbic acid using graphitic carbon nitride nanosheets as a fluorescent “switch”. Biosens. Bioelectron. 2015, 68, 210−217. (30) Guo, X.; Yue, G.; Huang, J.; Liu, C.; Zeng, Q.; Wang, L. LabelFree Simultaneous Analysis of Fe(III) and Ascorbic Acid Using Fluorescence Switching of Ultrathin Graphitic Carbon Nitride Nanosheets. ACS Appl. Mater. Interfaces 2018, 10, 26118−26127. (31) Shao, N.; Zhang, Y.; Cheung, S.; Yang, R.; Chan, W.; Mo, T.; Li, K.; Liu, F. Copper Ion-Selective Fluorescent Sensor Based on the Inner Filter Effect Using a Spiropyran Derivative. Anal. Chem. 2005, 77, 7294−7303. (32) Yang, M.; Tang, Q.; Meng, Y.; Liu, J.; Feng, T.; Zhao, X.; Zhu, S.; Yu, W.; Yang, B. Reversible “Off−On” Fluorescence of Zn2+Passivated Carbon Dots: Mechanism and Potential for the Detection of EDTA and Zn2+. Langmuir 2018, 34, 7767−7775. (33) Sarkar, S.; Shunmugam, R. Unusual Red Shift of the Sensor While Detecting the Presence of Cd2+ in Aqueous Environment. ACS Appl. Mater. Interfaces 2013, 5, 7379−7383. (34) Gunnlaugsson, T.; Lee, T. C.; Parkesh, R. Cd(II) Sensing in Water Using Novel Aromatic Iminodiacetate Based Fluorescent Chemosensors. Org. Lett. 2003, 5, 4065−4068. (35) Kong, D.; Yan, F.; Han, Z.; Xu, J.; Guo, X.; Chen, L. Cobalt(ii) ions detection using carbon dots as an sensitive and selective fluorescent probe. RSC Adv. 2016, 6, 67481−67487. (36) Paustenbach, D.; Finley, B.; Mowat, F.; Kerger, B. Human Health Risk and Exposure Assessment of Chromium (VI) in Tap Water. J. Toxicol. Environ. Health, Part A 2003, 66, 1295−1339. (37) Zhitkovich, A. Chromium in Drinking Water: Sources, Metabolism, and Cancer Risks. Chem. Res. Toxicol. 2011, 24, 1617− 1629. (38) Zhang, Y.; Fang, X.; Zhao, H.; Li, Z. A highly sensitive and selective detection of Cr(VI) and ascorbic acid based on nitrogendoped carbon dots. Talanta 2018, 181, 318−325. (39) Xu, X.-R.; Li, H.-B.; Li, X.-Y.; Gu, J.-D. Reduction of hexavalent chromium by ascorbic acid in aqueous solutions. Chemosphere 2004, 57, 609−613. (40) Bhati, A.; Anand, S. R.; Gunture; Garg, A. K.; Khare, P.; Sonkar, S. K. Sunlight-Induced Photocatalytic Degradation of Pollutant Dye by Highly Fluorescent Red-Emitting Mg-N-Embedded Carbon Dots. ACS Sustainable Chem. Eng. 2018, 6, 9246−9256. (41) Suryawanshi, A.; Biswal, M.; Mhamane, D.; Gokhale, R.; Patil, S.; Guin, D.; Ogale, S. Large scale synthesis of graphene quantum dots (GQDs) from waste biomass and their use as an efficient and selective photoluminescence on−off−on probe for Ag+ ions. Nanoscale 2014, 6, 11664−11670. (42) Wang, Z.; Yu, J.; Zhang, X.; Li, N.; Liu, B.; Li, Y.; Wang, Y.; Wang, W.; Li, Y.; Zhang, L.; Dissanayake, S.; Suib, S. L.; Sun, L. Large-Scale and Controllable Synthesis of Graphene Quantum Dots from Rice Husk Biomass: A Comprehensive Utilization Strategy. ACS Appl. Mater. Interfaces 2016, 8, 1434−1439. 1590
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
ACS Omega
Article
(43) Zhang, Z.; Zheng, T.; et al. Progress of Carbon Quantum Dots in Photocatalysis Applications. Part. Part. Syst. Charact. 2016, 33, 457−472. (44) Zheng, M.; Xie, Z.; Qu, D.; Li, D.; Du, P.; Jing, X.; Sun, Z. On− Off−On Fluorescent Carbon Dot Nanosensor for Recognition of Chromium(VI) and Ascorbic Acid Based on the Inner Filter Effect. ACS Appl. Mater. Interfaces 2013, 5, 13242−13247. (45) Roy, P.; Chen, P.-C.; Periasamy, A. P.; Chen, Y.-N.; Chang, H.T. Photoluminescent carbon nanodots: synthesis, physicochemical properties and analytical applications. Mater. Today 2015, 18, 447− 458. (46) Rong, M.; Feng, Y.; Wang, Y.; Chen, X. One-pot solid phase pyrolysis synthesis of nitrogen-doped carbon dots for Fe3+ sensing and bioimaging. Sens. Actuators, B 2017, 245, 868−874. (47) Ju, J.; Chen, W. Synthesis of highly fluorescent nitrogen-doped graphene quantum dots for sensitive, label-free detection of Fe (III) in aqueous media. Biosens. Bioelectron. 2014, 58, 219−225. (48) Han, C.; Wang, R.; Wang, K.; Xu, H.; Sui, M.; Li, J.; Xu, K. Highly fluorescent carbon dots as selective and sensitive “on-off-on” probes for iron(III) ion and apoferritin detection and imaging in living cells. Biosens. Bioelectron. 2016, 83, 229−236. (49) Tripathi, K. M.; Singh, A.; Bhati, A.; Sarkar, S.; Sonkar, S. K. Sustainable Feasibility of the Environmental Pollutant Soot to FewLayer Photoluminescent Graphene Nanosheets for Multifunctional Applications. ACS Sustainable Chem. Eng. 2016, 4 (12), 6399−6408. (50) Capellmann, M.; Bolt, H. M. Chromium (VI) reducing capacity of ascorbic acid and of human plasma in vitro. Arch. Toxicol. 1992, 66, 45−50. (51) Gong, X.; Liu, Y.; Yang, Z.; Shuang, S.; Zhang, Z.; Dong, C. An “on-off-on” fluorescent nanoprobe for recognition of chromium(VI) and ascorbic acid based on phosphorus/nitrogen dual-doped carbon quantum dot. Anal. Chim. Acta 2017, 968, 85−96. (52) Oliveira, L. F.; Canevari, N. T.; Guerra, M. B. B.; Pereira, F. M. V.; Schaefer, C. E. G. R.; Pereira-Filho, E. R. Proposition of a simple method for chromium (VI) determination in soils from remote places applying digital images: A case study from Brazilian Antarctic Station. Microchem. J. 2013, 109, 165−169. (53) Pflaum, R. T.; Howick, L. C. The Chromium-Diphenylcarbazide Reaction1. J. Am. Chem. Soc. 1956, 78, 4862−4866. (54) Kumar, A. S. K.; Rajesh, N. Exploring the interesting interaction between graphene oxide, Aliquat-336 (a room temperature ionic liquid) and chromium(VI) for wastewater treatment. RSC Adv. 2013, 3, 2697−2709. (55) Zhu, H.; Fan, J.; Wang, B.; Peng, X. Fluorescent, MRI, and colorimetric chemical sensors for the first-row d-block metal ions. Chem. Soc. Rev. 2015, 44, 4337−4366. (56) Wang, Z.; Bush, R. T.; Sullivan, L. A.; Liu, J. Simultaneous Redox Conversion of Chromium(VI) and Arsenic(III) under Acidic Conditions. Environ. Sci. Technol. 2013, 47, 6486−6492. (57) Zhang, L.; Lay, P. A. EPR Spectroscopic Studies of the Reactions of Cr(VI) with l-Ascorbic Acid, l-Dehydroascorbic Acid, and 5,6-O-Isopropylidene-l-ascorbic Acid in Water.1 Implications for Chromium(VI) Genotoxicity. J. Am. Chem. Soc. 1996, 118, 12624− 12637. (58) Malaviya, P.; Singh, A. Physicochemical Technologies for Remediation of Chromium-Containing Waters and Wastewaters. Crit. Rev. Environ. Sci. Technol. 2011, 41, 1111−1172.
1591
DOI: 10.1021/acsomega.8b03191 ACS Omega 2019, 4, 1581−1591
