Carbon nanopowder directed synthesis of carbon dots for sensing multiple targets

Accepted Manuscript Title: Carbon nanopowder directed synthesis of carbon dots for sensing multiple targets Authors: Qian Su, Xiaodong Wei, Jingxin Mao, Xiaoming Yang PII: DOI: Reference:

S0927-7757(18)31370-0 https://doi.org/10.1016/j.colsurfa.2018.11.015 COLSUA 22985

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

6 October 2018 30 October 2018 7 November 2018

Please cite this article as: Su Q, Wei X, Mao J, Yang X, Carbon nanopowder directed synthesis of carbon dots for sensing multiple targets, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.11.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Carbon nanopowder directed synthesis of carbon dots for sensing multiple targets Qian Su, Xiaodong Wei, Jingxin Mao, Xiaoming Yang College of Pharmace utical Sciences, Key Laboratory of Luminescent and Real-Time

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Analytical Chemistry (Ministry of Education), Southwest University, Chongqing

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400715, China

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Graphical abstract

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Hereby, multi-function carbon dots (CDs) derived from carbon nanopowder have been originally synthesized, and possessed the specific response ability for three related targets (temperature, Ni2+ and doxycycline). To be specific, the fluorescence of CDs was increased while being heated. Meanwhile, obvious quenching by Ni2+ confered CDs the ability of sensing Ni2+. In addition, the fluorescent color of CDs obtained here turned green from blue with the introduction of doxycycline, and thus an optical detection for doxycycline was proposed.

* To whom correspondence should be addressed. Tel: 86-23-68251225; Fax: 86-23-68251225; E-mail: [email protected]

Highlights 

We creatively utilized carbon nanopowder to synthesize multi-function carbon dots.

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The proposed CDs were employed for sensing multiple targets by different routes.

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This CDs show the promising of functioning as a nanoscale thermometer.

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Abstract

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Carbon nanopowder usually serves as the precursor to prepare the ultra-compact probes by the surface passivation, however, synthesizing the fluorescent sensors based on this type of nanomaterial remains challenging. Herein, Carbon nanopowder has

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been originally employed for synthesizing the multi-function carbon dots through a

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one-step hydrothermal treatment method. Importantly, the current CDs can effectively

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function as a nanoscale thermometer on the basis of their temperature-induced

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fluorescence enhancement, which allows a wide linear range from 25 ℃ to 95 ℃. In

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addition, the CDs yield a response towards nickel (II) by their specific fluorescence

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quenching, and thus achieves a detection range from 80 to 6000 M. Again, the fluorescent color of the CDs turned green from blue while doxycycline was

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introduced, and the visual evaluation of doxycycline within a range from 10 to 1000 M has been obtained, thereby providing an innovative way of detecting doxycycline.

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On the whole, the proposed CDs exhibit the multi-function of sensing three targets (temperature, Ni2+ and doxycycline) by virtue of three different routes.

Key words: Carbon dots; Temperature sensing; Assaying; Nickel (II); Doxycycline

1. Introduction Being a sort of fluorescent carbon nanomaterials, carbon dots (CDs) have harvested much interest and served as the possible alternative to the orthodox

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semiconductor quantum dots [1]. Semiconductor quantum dots are apparently limited by high toxicity, owing to the addition of heavy metals during their preparations

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[2,3,4]. The advantage becomes a driving force for the current CDs to take the place

of semiconductor quantum dots, because CDs shows the analogic fluorescence

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characteristics as well as hypotoxicity, low cost and acceptable biocompatibility [1].

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CDs, consequently, have been paid extensive attention in the terms of the following

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fields including biological labeling, sensing, nanobiotechnology, biomedicine,

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delivering drugs and photocatalysis [5,6,7], etc. In the past several years, two primary

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strategies have been proposed to prepare carbon dots. In particular, one is defined as “top down”, mainly composed of acidic oxidation, laser ablation, arc discharge and

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electrochemical oxidation. Another is considered as “bottom up”, microwave,

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hydrothermal and ultrasonic inclusive, and widely used for the synthesis of carbon dots [8].

Being one of basic thermodynamic variables, temperature is of great essence to

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contribute to the biochemical and physiological procedure [9]. However, few CDs-based protocols have been reported on temperature sensors. Meaningfully, the favorable biocompatibility potentiates CDs to measure the temperature and its variations in a living cell, which is considerable for cell biology and biomedicine [10].

As a member of representative heavy metals, the importance of nickel has been cognizant, because it functions as an indispensable nutrient related to multifarious biochemical and physiological processes [11]. Howbeit, the inhalation of nickel and its corresponding compounds could lead to various severe illnesses, like respiratory

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system cancer, even though nickel is of lower toxicity than other metallic ions [12]. Besides, excessive nickel may also induce kinds of severe pathema, including

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pneumonitis, asthma, central nervous system disorders and respiratory system cancer

[12]. Hence, developing efficient methodologies for assaying nickel (II) ion are

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essential. There have existed several methods including atomic absorption

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spectrometry, inductively coupled plasma mass spectroscopy, X-ray fluorescence

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spectrometry, neutron activation analysis, atomic fluorescence spectrometry, and

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anodic stripping voltammetry for detecting nickel (II) in environmental and biological

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specimens [13-18]. Among them, the fluorescent assays are favorable for their high sensitivity, speedy analysis and non-destruction to the sample [19].

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Additionally, one of the most crucial antibiotics are tetracyclines, which act on

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Gram-positive and Gram-negative germs. To be more precise, this series of antibiotics could inhibit the protein synthesis in bacteria so that they lose the ability of multiplication. In particular, doxycycline is a member of tetracyclines consisting of

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the structural formula of β-diketonate, and shows the broad antimicrobial spectrum. As a result of its pretty reliable absorption and fairly long half-life, thus permitting less frequent administration, doxycycline is commonly used to treat the specific infections [20]. Consequently, it is often introduced clinically to cure syphilis, pelvic

inflammatory and sinusitis [21]. Therefore, detecting doxycycline towards various purposes is of great importance. Recently, there existed kinds of multifunctional carbon dots applied for detection, ion sensors and fluorescent inks [22-25]. Also, carbon nanopowder has been reported

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as the precursor to prepare the carbon dots, for instance, the ultra-compact fluorescent probe was raised while 2,20-(ethylenedioxy)-bis(ethylamine) (EDA) -molecules

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played the role of passivating the carbon particle surfaces [26], whereas it cost more than four days and the organic solvents were introduced. Here, multi-function carbon

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dots were originally synthesized by one-step hydrothermal treatment at 220 ℃, while

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carbon nanopowder served as the precursors. Meanwhile, the whole preparation

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process was basically environment-friendly, stimulating the CDs’ biocompatibility

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and wider applications. Also, the morphological characteristics, typical functional

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groups, and photochemical properties of CDs were systematically explored. Importantly, the proposed CDs can function as both a nano-thermometer and a

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fluorescent probe for assaying nickel (II) and doxycycline (Fig. 1). As a temperature

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sensor, the fluorescence intensity of CDs linearly increases within the range from 25 ℃ to 95 ℃. While being a fluorescent probe, CDs exhibit the satisfactory linear relationships for sensing nickel (II) and doxycycline. Specifically, the fluorescence

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emission peak of CDs was conspicuously quenched once nickel (II) was added, while CDs showed little change with the additions of other metal ions. In sharp contrast, a fluorescent emission peak at 500 nm clearly appears when doxycycline was introduced (Ex=400 nm), providing a new way for optically detecting doxycycline.

Taking the advantage of the multi-response for temperature, nickel (II) and doxycycline, the CDs synthesized here have broadened the avenues for investigating

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the three targets.

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2. Materials and methods

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Fig. 1 (A) Schematic illustration of synthesizing CDs; (B) Description of sensing temperature, nickel (II) and doxycycline.

2.1 Chemicals

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Carbon nanopowder (30 nm, purity 99.5+%) was obtained from Aladdin Co., Ltd.

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(Shanghai, China). Metal ions (e.g. NiCl2, CoCl2, MnCl2, ZnCl2, PbBr2, BaSO4, CaCl2, HgCl2, SrCl2 · 6H2O, CuSO4, KCl, and AlCl3) were obtained from Chron Chemicals Co., Ltd. (Chengdu, China). Hydrazine hydrate (80%, N2H4·H2O) were

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purchased from Chuandong Chemical Industry Co., Ltd. (Chongqing, China). Lincomycin

hydrochloride,

chlortetracycline

hydrochloride,

tetracycline

and

doxycycline were obtained from Sangon Biotech Co., Ltd. (Shanghai, China). Folic acid was obtained from Macklin Co., Ltd. (Shanghai, China). Ultrapure water, 18.25

MΩ.cm, provided with an Aquapro AWL-0502-P ultrapure water system (Chongqing, China) was used for all the following experiments. 2.2 Instrumentation All the fluorometric assays were achieved on a Hitachi F-7000 fluorescence

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spectrophotometer (Tokyo, Japan). The slit width was set at 5 nm and 10 nm for excitation and emission in 1 cm × 1 cm quartz cells. Simultaneously, the absorption

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spectrum was captured on a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan).

Again, the size distribution was conducted with a Zetasizer Nano Dynamic light

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scattering (DLS) instrument (Malvern, England). High-resolution transmission

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electron microscopy (HR-TEM) images were obtained with a TECNAI G2 F20

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microscope (Columbia, America) at 200 kV. Elements and functional groups analyses

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were performed with ESCALAB 250 X-ray photoelectron spectrometer (XPS) and

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Fourier transform infrared (FTIR) spectrometer (Tokyo, Japan), separately. The powder of CDs was acquired on lyophilization in PiloFD8-4.3V (Charlotte, USA). An

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Olympus E-510 digital camera (Tokyo, Japan) was employed to obtain the

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photographs. A Fangzhong pHS-3C digital pH meter (Chengdu, China) was used for identifying the pH values of the solutions and a vortex mixer QL-901 (Haimen, China) was applied to blend the solutions. The water bath (DF-101s) by Gongyi

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Experimental Instruments Factory (Gongyi, China) was used for providing varied temperature. The electric heating drying oven (DHG-9140A) from Keelrein instrument Co., Ltd. (Shanghai, China) was applied for the synthetic reactions. The centrifuge (TG16-W) of Cence instrument Co., Ltd. (Hunan, China) was employed for

separations. A series of experiments were performed at room temperature, if not mentioned. 2.3 Synthesis of CDs The CDs composed of carbon nanopowder were innovatively synthesized by

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one-step hydrothermal treatment. Briefly, 0.06 g of carbon nanopowder, 6 mL of NaOH (1.5 M) and 500 L hydrazine hydrate (80%, N2H4·H2O) were successively

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added into an autoclave of 25 mL. Followed by heating at 220 ℃ for 10 hours, the

autoclave reached the room temperature, and the reaction solution was centrifuged at

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5500 rpm for 2 min. The colorless supernatant was gathered. Subsequently, the

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previous supernatant was filtered with 0.22 m micropore membrane to discard the

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large particles and then was subjected to dialysis by using a dialysis membrane (100

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MWCO) for 24 h. Ultimately, the white powder of CDs was collected through

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lyophilizing and further dissolved in the ultrapure water with the final concentration of 0.75 mg/mL. The prepared CDs were stored in the dark at 4 ℃ for the future using.

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2.3 Temperature sensing

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Typically, 100 L of CDs solution (0.75 mg/mL) was introduced into a 1.5-mL centrifuge tube, and diluted to 1 mL with ultrapure water. Then, the tube was kept in a water bath at gradually increased temperatures, each temperature node for 15 min.

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Finally, a series of fluorescence intensities of CDs at 25, 35, 40, 45, 65, 80, 95 ℃ were respectively recorded. 2.4 Fluorescent assay of Ni2+ Basically, the detection of Ni2+ was performed in ultrapure water. Primarily, 300

L of CDs (0.75 mg/mL) and 600 L of ultrapure water were successively pipetted into a 1.5-mL vial. Subsequently, 100 L of Ni2+ with different concentrations (80, 200, 600, 1000, 2000, 4000, 6000 M) were separately added. With the vortex for 60s, the vials were incubated at 25 ℃ for 5 min and the fluorescence spectra of the

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mixtures were obtained. To evaluate the selectivity of the sensing approach, Ni2+, Co2+, Mn2+, Zn2+, Pb2+, Ba2+, Ca2+, Hg2+, Sr2+, Cu2+, K+, Na+ and Al3+ (2.5 mM, 100

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L), totally 13 various salt cations, were introduced into CDs with the identical conditions.

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2.5 Fluorescent assay of doxycycline

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Initially, 400 L of CDs (0.75 mg/mL) and 500 L of ultrapure water were added

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into a 1.5-mL vial in succession. Then, 100 L of doxycycline solutions with diverse

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concentrations (10, 100, 200, 400, 600, 800 and 1000 M) were separately introduced.

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At last, all the above solutions were incubated for 5 min at 25 ℃ and followed by the fluorometric analysis (Ex=400 nm). To identify their selectivity of the CDs, folic acid,

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lincomycin hydrochloride, chlortetracycline hydrochloride, tetracycline, doxycycline

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(1 mM, 100 L) were also respectively introduced with the equal conditions. 3. Results and discussion

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3.1 Characterization of CDs To investigate the fluorescence mechanism of the as-prepared CDs, the specific

characterizations have been studied detailedly. Generally, the CDs obviously exhibited the blue fluorescence with the maximum emission peak at 430 nm while excited at 310 nm (Fig. 2A). Meanwhile, the solution of CDs emitted the strong blue

fluorescence (Fig. 2A, picture b) with UV light of 365 nm, while being almost colorless under daylight (Fig. 2A, picture a). Additionally, it could be observed that there were three peaks in the UV-Vis spectrum (Fig. S1, ESI†), where the two peaks at 220 and 227 nm were separately attributed to the π-π* transitions of C=C and C=N,

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and the peak of 290 nm was induced by an n-π* transition of C=O [27,28,29]. Furthermore, Fig. 2B indicated a clear HR-TEM image of the CDs, which was applied

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to survey the morphology and particle distributions, and the current CDs showed the well dispersity. Besides, the diameter of CDs was investigated by DLS, demonstrating

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that the CDs ranged from 1.12 nm to 4.19 nm (Fig. 2B). When the excitation

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wavelengths of CDs varied from 310 nm to 410 nm, the emission peak showed the red

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shift and the fluorescence intensity gradually reduced (Fig. 2D), illustrating the

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excitation-dependent behavior of the CDs.

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In addition, FTIR and XPS spectra were utilized to identify the surface groups, elemental components and chemical structure of the CDs. Basically, FTIR spectrum

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was applied for analyzing the surface functional groups of the proposed CDs.

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Particularly, the FTIR spectrum indicated the CDs with the ample polar groups, for instance, the stretching vibrations of O-H/N-H at 3430 cm-1 and C=O at 1655 cm-1, concluding that the carboxyl and amino existed (Fig. 2C). Simultaneously, the

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stretching vibration of C=N/C=C at 1560 cm-1 demonstrated that the N-doped polyaromatic structures of the CDs formed by virtue of the reaction [29] (Fig. 2C). To clarify the elementary components and chemical bonds of the CDs as-synthesized, the XPS spectra were obtained (Fig. 3A). As shown in Figure 3, the XPS spectra of the

CDs supported the FTIR data. Fundamentally, the XPS survey showed three major peaks at 285.1, 401.1, and 532.1 eV for C1s, N1s, and O1s, separately (Fig. 3A), indicating that the proposed CDs primarily consisted of C (29.47%), O (68.69%) and N (1.84%) (Table S1, ESI†). More specifically, the high resolution XPS survey of C1s

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revealed three peaks at 284.6 eV, 285 eV and 288.9 eV (Fig. 3B), assigning to C=C, C-N, and C=O apart on surfaces of CDs, in accordance with the former FTIR.

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Simultaneously, the N1s spectrum (Fig. 3C) displayed two peaks at 399.3 and 400.3 eV, attributing to pyrrolic N (H-N-(C)2) and graphitic N (N-(C)3), respectively. Again,

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three peaks at 535.6 eV, 532.0 eV, and 530.9 eV of the O1s survey (Fig. 3D) was

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originated from the bands of H-O-H, C-OH, C-O-C and C=O. Taken together, the

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XPS survey agreed with that of FTIR, suggesting the as-prepared CDs showed the

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multiple polar functional groups such as -OH, -COOH, C=O and -NH2 [29,30].

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Meanwhile, the previous UV-vis, FTIR and XPS spectra indicated that the CDs were composed of the π-conjugated domains [31]. Besides, a variety of -OH groups on the

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surface of CDs provided their high polarity and water dispersibility [32]. Usually,

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both the size effect and surface defects play the major roles of regulating the fluorescent emission of CDs. Here, the bulk carbon nanopowder was cut into small pieces while heated in the alkaline environment, and their surfaces were modified by

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oxygen-based groups. Consequently, the surface state emissive trap of carbon nanopowder formed during the reaction processes, and thus leading to the blue-emission [33].

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Fig. 2 (A) Fluorescence excitation and emission spectra of CDs. Inset: pictures of CDs under visible (a) and UV light (b); (B) HR-TEM image. Inset: the size-distributions of CDs; (C) FTIR spectrum of CDs; (D) Emission spectra of CDs for varying excitation wavelengths.

Fig. 3 (A) XPS survey of the CDs. (B–D) High-resolution C1s, N1s and O1s XPS spectra of the CDs, respectively.

3.2 Optimizing the synthesis conditions of CDs To investigate the optimization conditions for preparing CDs, a battery of tests were conducted. As illustrated in Fig. S2 (ESI†), the volumes of N2H4·H2O and temperature strongly affected the fluorescence intensity of the CDs mainly containing

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carbon nanopowder, while the concentrations of NaOH and reaction time slightly change the fluorescence intensity, suggesting the preparation of CDs relied on the four

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alterable conditions. Consequently, 6 mL of 1.5 M NaOH, 500 L of N2H4·H2O, 10 hours and 220 ℃ are identified as the preferred conditions to synthesize the CDs with

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carbon nanopowder.

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3.3 Stability of CDs

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Initially, to explore the fluorescence stability of the as-synthesized CDs for

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tolerating circumstances, the associated experiments were performed. As illustrated in

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Fig. S3C and S3D (ESI†), there was no apparent change for the fluorescent intensity with different concentrations of NaCl aqueous solutions (up to 1 M) and time (under

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the irradiation of 365 nm UV), displaying the acceptable fluorescence stability and

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resistance of the current CDs towards the high ionic strength and photobleaching. However, the fluorescent intensity of CDs couldn’t keep steady when pH and temperature varied, suggesting that pH and temperature displayed obvious effects on

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this type of CDs (Fig. S3A, S3B, ESI†). To further clarify the properties of CDs, the solid powder of CDs was acquired by lyophilization and emitted the striking fluorescence with UV light (Fig. S4A, ESI†), whereas being as white powder under

daylight and totally different from the black carbon nanopowder, which further confirmed the successful synthesis of CDs (Fig. S4B, ESI†). 3.3 Temperature sensing Interestingly, the fluorescence intensity of CDs showed a dramatically

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temperature-dependent property. As described in Fig. 4A, the fluorescence intensities increased monotonously by raising the temperature within the range of 25-95 ℃,

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demonstrating that these CDs showed the potential of serving as a temperature sensor.

Simultaneously, their emission peaks showed little shift, indirectly revealing that the

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high-purity of CDs were produced (Fig. 4A). Importantly, an acceptable linear

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relationship emerged with the range of 25 ℃ to 95 ℃ (Fig. 4B).

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F = 634.9 + 6.158 T (R2 = 0.998, T: 25 ℃-95 ℃)

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For this equation, F stands for the fluorescent intensity as well as T representing the

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temperature. Basically, most of the previously reported thermometers show the temperature-induced fluorescence quenching, and the current phenomenon was

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different [34-37]. For the mechanism, the as-obtained CDs were equipped with

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multiple carboxyl and hydroxyl groups, which could enhance their water dispersibility and stability [38]. More importantly, these groups played an essential role for aggregation-induced emission enhancement (AIEE), since the further aggregations

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may be blocked by the surface groups of CDs to avoid the larger particles appearing [39,40]. As a result, the aggregations of CDs occurring here caused the enhancement of fluorescence with the raised temperature rather than the quenched fluorescence [41].

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Fig.4. (A) Fluorescence spectra of CDs in different temperatures; (B) Plot of the fluorescence intensity versus the temperature.

3.4 Detection of Ni2+

To explore the functions of the prepared CDs, various metal ions have been

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introduced. Meaningfully, it was observed that Ni2+ could obviously quench the

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fluorescence of CDs (Fig. 5A), indicating that this type of CDs could serve as a

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fluorescent probe to assay Ni2+. Towards the specificity of CDs for Ni2+ detection, the fluorescence intensities responding to 12 other interfering metal ions including Co2+,

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Mn2+, Zn2+, Pb2+, Ba2+, Ca2+, Hg2+, Sr2+, Cu2+, K+, Na+ and Al3+ were respectively

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recorded with the speedy reaction time of 5 min. As shown in Fig. 5B, only Ni2+ could result in the striking decline of the fluorescent intensity except a slight influence by

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Mn2+ (Fig. 5B), demonstrating the satisfactory specificity of the CDs for sensing Ni2+. Accordingly, the calibration curves of Ni2+ are established. To be more specific,

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the fluorescent emission peaks of CDs decreased versus the logarithmic plots of Ni2+ concentrations (80, 200, 600, 1000, 2000, 4000, 6000 M), and its correlation coefficient of linear regression was 0.992. Meanwhile, the detection limit was calculated as 16.57 M based on the signal-to-noise ratio of 3 (Fig. 5C and 5D). Towards the mechanism, the fluorescence quenching of CDs could be related to the

groups of -COOH, -OH, -NH2 on their surfaces, which possibly function as the receptor like Schiff base. Consequently, when Ni2+ interacted with the receptor structure on the surface of CDs, the charge transferring of the CDs were destroyed,

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and resulting in their fluorescence change [42].

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Fig. 5. (A) Fluorescence spectra of CDs, Ni2+ and CDs + Ni2+, Inset: images for 600 L of 0.75 mg/mL CDs (a), and CDs with 1mM of Ni2+ (b); (B) Selective tests of CDs with different metal ions (F0 and F are the fluorescence intensities of CDs at 430 nm without and with 250 M metal ions respectively); (C) Fluorescence spectra of CDs in the presence of varied concentrations of Ni2+; (D) Plot of the fluorescence intensity versus the logarithmic concentrations of Ni2+.

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3.5 Detection of doxycycline Taking advantage of the favorable fluorescent properties of CDs, the visual, fast,

stable and low-cost detection procedure of doxycycline was built up. Particularly, doxycycline showed an evident response to the fluorescence of the CDs, and their color varied from blue to green (Fig. 6A, photograph a, b). Besides, the florescent

detection of doxycycline by CDs was performed under the excitation wavelength of 400 nm (Fig. 6A). Regarding to the mechanism, C=C groups of Doxycycline-CDs increased (Fig. S6), and the formation of conjugated system elevated the energy level of π state, and decreased the energy levels between π–π* energy gaps, thus leading to

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the emission-wavelength shifting from 430 nm to 500 nm [20]. Meanwhile, the larger conjugated-π system formed as well as the concentration of doxycycline increasing,

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and showing the higher intensity. To identify the specificity of the proposed approach,

the fluorescence response of CDs in the presence of various analogues (folic acid,

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lincomycin hydrochloride, chlortetracycline hydrochloride, tetracycline, 100 M for

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each) was studied. As illustrated in Fig. 6B, the analogues scarcely exhibited effect on

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used for selectively sensing doxycycline.

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detecting doxycycline with the same conditions, suggesting that the CDs could also be

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By virtue of the present sensing model, the linear calibration curve was built up by adding various concentrations of doxycycline including 10, 100, 200, 400, 600,

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800 and 1000 M (Fig. 6C). Again, when Ni2+ was gradually introduced into the CDs

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solutions by raising the concentrations up to 1000 M, the related fluorescent intensities increased accordingly (Fig. 6C), accompanying the color of CDs changing from blue to green (Fig. 6D). In addition, F increase versus the diverse concentrations

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of doxycycline displayed a linear range of 10-1000 M with the correlation coefficient of 0.994, and the detection limit was 16.35 M obtained, showing the potential of the as-prepared CDs for detecting doxycycline.

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4. Conclusion

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Fig. 6. (A) Fluorescence spectra of CDs, doxycycline and both of them, Inset: images for 600 L of 0.75 mg/mL CDs (a), and CDs with 100 M of doxycycline (b); (B) Comparison of fluorescence intensities of CDs in the presence of folic acid, lincomycin hydrochloride, chlortetracycline hydrochloride, tetracycline, doxycycline, each for 100 M; (C) Fluorescence spectra of CDs in the presence of various concentrations of doxycycline; (D) Plot of the fluorescence intensity versus the concentrations of doxycycline. Inset: the corresponding images under UV light of 365 nm.

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In summary, one-step straightforward hydrothermal method has been raised to synthesize the multi-function CDs derived from carbon nanopowder. Endowed the

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excellent water-solubility, chemical and optical stability, the CDs here showed the broad prospects for the utilization of chemical sensing. In this study, the CDs have

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been successfully applied for the quantitative determinations of temperature, Ni2+ and doxycycline with the satisfactory accuracy. Importantly, the fluorescence intensity of CDs linearly increased with the rising temperature within the range from 25 ℃ to 95 ℃. Based on Ni2+-induced fluorescence quenching of the CDs, an acceptable linear regression for detecting Ni2+ was achieved over the wide concentration range from 80

to 6000 M. Moreover, the fluorescent signal of CDs gradually turned green from blue with the increasing concentrations of doxycycline, and thus a visual method for doxycycline detection was proposed. For the facile preparation of the stable and multi-function CDs, this study may broaden a potential avenue for the application of

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CDs in the field of chemical sensing and beyond.

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Acknowledgments

We gratefully acknowledge the financial support by Fundamental Research Funds

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for the Central Universities (XDJK2018AC004), Natural Science Foundation of

on

Designing

Screening

Drug

Candidates

of

Chongqing

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(cstc2015zdcy-ztzx120003).

and

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Project

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Chongqing (cstc2018jcyjAX0197), and Fundamental Research Funds for Innovative

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