Presence of fluorescent carbon nanoparticles in baked lamb: Their

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Presence of fluorescent carbon nanoparticles in baked lamb: Their properties and potential application for sensor Haitao Wang, Yisha Xie, Shan Liu, Shuang Cong, Yukun Song, Xianbing Xu, and Mingqian Tan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b02913 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Journal of Agricultural and Food Chemistry

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Presence of fluorescent carbon nanoparticles in baked lamb: Their properties

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and potential application for sensor

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Haitao Wang,a,b,c Yisha Xie,a,b,c Shan Liu,a,b,c Shuang Cong,a,b,c Yukun Song,a,b,c

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Xianbing Xu,a,b,c MingqianTana,b,c*

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a

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Liaoning 116034, People’s Republic of China

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b

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People’s Republic of China

School of Food Science and Technology, Dalian Polytechnic University, Dalian,

National Engineering Research Center of Seafood, Dalian, Liaoning 116034,

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c

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Liaoning 116034, People’s Republic of China

Engineering Research Center of Seafood of Ministry of Education of China, Dalian,

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* To whom correspondence should be addressed. E-mail: M. Tan, [email protected],

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ORCID: 0000000275350035.

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ACS Paragon Plus Environment


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Abstract

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The presence of nanoparticles in food has drawn much attention in recent years.

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Fluorescent carbon nanoparticles are a new class of nanostructures, however, the

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distribution and physicochemical properties of such nanoparticles in food remains

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unclear. Herein, the presence of fluorescent carbon nanoparticles in baked lamb was

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confirmed and their physicochemical properties were investigated. The fluorescent

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carbon nanoparticles from baked lamb emit strong blue fluorescence under ultraviolet

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light with a 10% fluorescent quantum yield. The nanoparticles are roughly spherical

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in appearance with a diameter of around 2.0 nm. Hydroxyl, amino, and carboxyl

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groups exist on the surface of nanoparticles. In addition, the nanoparticles could serve

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as fluorescence sensor for glucose detection through an oxidation-reduction reaction.

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This work is the first report on fluorescent carbon nanoparticles present in baked lamb,

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which provides valuable insight into physicochemical properties of such nanoparticles

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and their potential application in sensors.

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Keywords

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Carbon dots; Baked lamb; Glucose; Sensor; Hydroxyl radical

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Introduction

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The use of artificial nanoparticles in food has drawn much attention in recent

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years owning to their benefits for food quality1 as well as potential adverse effects on

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human health2. In addition, nanoparticles are not confined to engineered materials in

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nanosize, but nanostructures are also present in food such as starch nanoparticles

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produced from gelatinized starch3. The public concerns about nanoparticles in food as

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well as the development of sophisticated analytical techniques provide an opportunity

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to detect nanomaterials that are directly derived from food products. Characterizing

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those nanostructures could provide both new information about nanoparticles in food

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and expand their potential application.

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Heating is the most widely used method for food processing both in the industry

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and at home4. High temperatures trigger interaction, such as Maillard reactions and

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pyrolysis, between food ingredients, which plays an important role in the formation

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and retention of food quality such as taste and color5.Simultaneously, carbon

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nanostructures may be generated in the process known as carbonization from organic

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compositions at high temperatures6, 7. Carbon nanoparticles can be obtained through

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hydrothermal carbonization process8 and pyrolysis process9. What’s more, microwave

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treatment is an effective method to generate carbon nanostructures as well10.

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Apparently, conditions for the formation of carbon nanoparticles mimic the heat

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processing of food. It is reasonable to propose that food components or even the

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complex food matrix could form carbon nanoparticles under routine thermal treatment

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through the carbonization process. Dai et al.7 reported the formation of



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nanometer-sized carbon dots from milk by microwave cooking. Al-Hadi et al.11

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discussed the formation of carbon nanostructures in bakery and metabolic stress in

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human mesenchymal stem cells induced by such nanoparticles.

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Carbon dots (CDs) are a new class of fluorescent nanomaterials, which generally

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refer to carbon-based fluorescent nanoparticles of near spherical geometry with sizes

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below 10 nm at least in one dimension, they are mainly composed of carbon, and

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other elements such as oxygen, hydrogen or nitrogen12-14. Since they were first

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discovered in 200415, CDs have been used extensively in various areas such as

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chemical sensing, biosensing, bioimaging and drug delivery due to their excellent

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fluorescence property, small size, low cost and high photostability16. Interestingly and

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importantly, the presence of CDs in food has rarely been studied. Sk et al.17 reported

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the presence of CDs in some carbohydrate-based food such as bread, jaggery, sugar

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caramel, corn flakes, and biscuits. The formation of these CDs was attributed to

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heating of the starting material.

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Meat is one of the important protein resources for human consumption and

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baking is a popular cooking method. It is reasonable to conjecture that the CDs may

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form through carbonaceous or self-assembled process under intensive heating during

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baking. The formation of graphene in meat during baking process has been

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demonstrated18. Fluorescent CDs were found in BBQ char of ground-beef which were

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cooked on a conventional outdoor grill19. However, to the best of our knowledge, the

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presence of CDs in baked lamb cooked in an electric oven has not been reported.

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In our previous work, the presence of CDs in instant coffee20, beer21, and


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commercial beverages22 has been shown as well as their potential applications in cell

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imaging and drug delivery. This encouraged us to search for CDs in food items,

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especially in protein-base food such as meat, in order to character their

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physicochemical properties as well as investigating their potential application. Herein,

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the presence of CDs in baked lamb was investigated for the first time and their

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properties were systematically studied, including their ﬂuorescence properties, surface

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chemistry, ﬂuorescence lifetime, and stability. In addition, their potential application

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in sensor was also explored by using glucose as an example.

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Experimental procedures

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Materials. Fresh lamb was purchased from a local grocery store. Hydrogen

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peroxide, ferrous sulfate, quinine sulfate, sodium chloride, glucose, glucose oxidase,

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5,5-dimethyl-1-pyrroline N-oxide (DMPO), and EDTA disodium salt were purchased

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from Aladdin technology (Shanghai, China). All other chemicals used in this work

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were of analytical grade. Millipore ultrapure water (Millipore, USA) was used

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throughout the experiments.

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Extraction of fluorescent nanoparticles. The baked lamb was carefully

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collected after being baked in an electric oven at 250 oC for 30 min. The fluorescent

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nanoparticles in the baked lamb were extracted by using ethanol for 24 h at room

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temperature with constant stirring. After removing large size particles by

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centrifugation, ethanol was removed by rotary evaporation and the products were

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dissolved in water and extracted with chloroform (1:4 by volume). The upper water

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phase was separated and extracted twice with chloroform to remove any hydrophobic



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impurity. After filtered through a 0.22 µm pore size membrane, the fluorescent

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nanoparticles in water phase were further purified by dialyzing against deionized

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water using a membrane with molecular weight cut-off 3500 Da for 2 days. The

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suspension of fluorescent nanoparticles was lyophilized and stored at 4 oC for further

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characterization and applications.

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Instrumentation. Transmission electron microscope (TEM) images were taken

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by using a JEM-2100 (JEOL, Tokyo, Japan) operation at 200 kV. Fluorescent spectra

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were obtained by using an F-2700 spectrophotometer (Hitachi, Tokyo, Japan) under

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ambient conditions. The ultraviolet−visible (UV−vis) spectra were obtained using a

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Lambda 35 UV−vis spectrophotometer (PerkinElmer, Waltham, USA). Fourier

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transform infrared (FT-IR) spectra were recorded by using a frontier FT-IR

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spectrometer (PerkinElmer, Waltham, USA) in KBr pellet with wavelength ranging

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from 500 to 4000 cm−1. The chemical composition was characterized by using a VG

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ESCALAB250 X-ray photoelectron spectrometer (XPS) and the C1s (284.8eV) was

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taking as the reference. Lifetime was measured by FLS980 spectrometer (Edinburgh

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Instruments, Edinburgh, UK) with a 270 nm laser as the excitation source.

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Fluorescence lifetime (τ) of the fluorescent nanoparticles was calculated using the

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following equation:

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τ = ( A1τ1 + A2 τ 2 + A3 τ 3 ) /( A1 + A2 + A3 )

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Where Ai is the fractional contributions of time-resolved decay lifetime of τi.

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Fluorescence quantum yield and lifetime measurement. The fluorescence quantum

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yield (QY) of nanoparticles was calculated by using quinine sulfate (dissolved in 0.1


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mol·L-1 H2SO4, QY=55%) as a reference. Fluorescence quantum yield (Φ) was

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calculated according to the following equation: ߔ = ߔ′ ⋅

‫ܣ‬ᇱ ‫݊ ܫ‬ଶ ⋅ ⋅ ‫ ܫ‬ᇱ ‫݊ ܣ‬ᇱ ଶ

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Where Φ is the QY of the sample, I is the sample’s integrated emission intensity,

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n is the refraction index (1.33 for water solutions), and A is the optical density. The

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ߔ′ refers to the QY of quinine sulfate. ‫ܣ‬ᇱ is the optical density of quinine sulfate,‫ܫ‬ᇱ is

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the integrated emission intensity of quinine sulfate, ݊ᇱ and nis the refraction index of

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quinine sulfate and testing sample, respectively. A series of solutions of fluorescent

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nanoparticles and quinine sulfate were prepared with optical absorbance values in the

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range of 0-0.1 at 370 nm. The photoluminescent spectra were recorded and their

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intensities integrated. QY was determined by comparison of the integrated fluorescent

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intensity vs absorbance curves. Both solutions were measured under the same

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instrumental condition.

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Electron spin resonance (ESR) spectroscopic measurements. Hydroxyl

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radicals were produced by a Fenton reaction (0.6 mmol·L-1EDTA-2Na-Fe2+ and 140

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mmol·L-1 H2O2 in 150 mmol·L-1 pH=7.4 phosphate buffer) and captured by DMPO

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(50 mmol·L-1). Control or test solution was placed in quartz capillary tubes for ESR

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measurement after being incubated at 40oC for 30 min. All the ESR measurements

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were carried out at ambient temperature using a Bruker A200 ESR spectrometer

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(Bruker BioSpin, Billerica, USA) with 0.721 mW microwave power, and 1 G field

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modulation.



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Detection of the hydrogen peroxide and glucose. For H2O2 detection, 40 µL

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carbon dots (5 mg·mL-1), 160 µL Fe2+ (10 mmol·L-1) and 400 µL H2O2 with different

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concentrations were added to 1400 µL Britton-Robinson (BR) buffer (pH=3). The

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mixture was shaken vigorously and incubated for 5 min at room temperature to ensure

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completion of the reaction. The fluorescence spectra were recorded on an F-2700

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fluorescence spectrophotometer at the excitation wavelength of 370 nm. The relation

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between the change of fluorescent intensity and concentration of H2O2 was plotted.

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For glucose detection, 10 µL 100 U·mL-1 glucose oxidase and 500 µL glucose

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with different concentrations were added to 500 µL phosphate buffer (0.1 mol·L-1

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pH=5.7) following incubation at 35 oC for 20 min. The reaction was terminated by

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adjusting the pH to 3.0 with HCl. Subsequently, 800 µL BR buffer (pH 3.0), 40 µL

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carbon dots (5 mg·mL-1), 160 µL 0.8 mmol·L-1 Fe2+ were added to the mixture. The

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fluorescence intensities were recorded.

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Analysis of glucose in real samples. The test beverages were diluted 100-folds

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with 100 mmol·L-1 phosphate buffer (pH 5.7) before fluorescence analysis. The

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glucose concentration in the beverages was measured as described above. In addition,

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glucose concentration was also determined on a DIONEX ICS-5000 ion

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chromatography equipped with a Dionex CarboPac PA1 column. The samples were

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diluted by using water and filtered through a 0.22 µm filter before analysis. The

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mobile phase was aqueous NaOH (250 mmol·L-1) at a flow rate of 0.25 mL·min-1.

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Results and discussion

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Presence of fluorescent nanoparticles in baked lamb. As the primary chemical


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component of meat, protein and fat might act as excellent substrates for chemical

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reactions during thermal processing. The formation of nanostructures from

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deteriorative carbonization of proteins mediated by a variety of thermal reactions has

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been demonstrated23, 24. Therefore, protein and fat from lamb may undergo complex

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reactions to form fluorescent nanoparticles during pyrolysis upon baking process. To

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prove this hypothesis, fluorescent nanoparticles were extracted from both the fresh

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lamb and baked lamb. The solutions obtained from baked lamb exhibited strong

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fluorescence under excitation at 365 nm while the solutions from fresh lamb exhibited

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no fluorescence (Figure S1). This result provided evidence for the presence of

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fluorescent nanoparticles in the baked lamb upon heating. In order to further prove the

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hypothesis, the solution obtained from the baked lamb was dropped on a

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carbon-coated copper grid for TEM observation. As expected, well dispersed and

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nearly spherical nanoparticles were observed (Figure 1A). The size distribution of the

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nanoparticles was relatively narrow, with the majority falling within 0.8-3.8 nm based

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on the statistical analysis of 100 particles and the average size was approximately 2.0

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nm (Figure 1B). Furthermore, discernible lattice structure was observed in high

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resolution TEM images as shown by inset of Figure 1A, which is similar to previously

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reported CDs25-27. Since fluorescence of organic molecules is invisible in TEM due to

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their low contrast, these results provided solid evidence for the presence of

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nanoparticles in the baked lamb. In addition, like most reported CDs26, 28, 29,30, the

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XRD patterns of the nanoparticles from baked lamb (Figure S2) only displayed a

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broader peak centered at around 25o revealing the amorphous nature of the



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nanoparticles probably due to the low carbonation rate under the baking conditions.

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This result provided additional evidence for the presence of nanoparticles in baked

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lamb.

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Optical properties of the nanoparticles purified from baked lamb. The

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UV-vis absorption and the steady-state fluorescent spectra were recorded to explore

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the optical properties of nanoparticles extracted from baked lamb. The absorption

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band at around 260 nm was observed in the UV-vis absorption spectra, which related

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to n–π* transitions of the aromatic C=C (Figure 1C). The nanoparticles emit

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fluorescence at 430−470 nm when excited in the range of 300−400 nm as shown in

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Figure 1C. The maximum fluorescent excitation and emission wavelengths of the

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nanoparticles were 370 and 426nm, respectively. It is important to note that a classical

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bathochromic emission phenomenon was observed, which is a characteristics of

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fluorescent CDs as in previous reports31. This complicated fluorescent behavior might

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be attributed to different emitting centers present in the CDs. Fluorescence decay

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profile of the nanoparticles dispersed in aqueous solution at room temperature can be

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fitted using a two-exponential function with an average lifetime of 8.05 ns (Figure1D),

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suggesting radiative recombination of the energy-trapping sites. The fluorescence QY

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of the nanoparticles was determined to be 10% (Figure S3, Table S1) using quinine

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sulfate in 0.1 mol·L-1 H2SO4 as a reference. The QY of nanoparticles was relatively

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high compared to previous results20, 32, the high fluorescence QY may be attributed to

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the well-ordered structure as observed by TEM.

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Surface analysis of the nanoparticles purified from baked lamb. The surface


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groups of nanoparticles were investigated by FT-IR. As shown in the Figure 2A, the

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peak at 3300 cm-1 was probably associated with the stretching vibration absorption

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peak of the –NH2 and –OH band. The peak at 2963 cm-1 corresponds to the C–H

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vibrations of methylene. The appearance of peaks near 1670 cm-1 were identified as

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the C=O stretching25. The peak at 1558 cm-1 originates from the C=C stretching. In

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addition, the peaks appeared around 1404 cm-1 may be caused by the symmetric

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stretching vibration of COOH33.

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X-ray photoelectron spectroscopy (XPS) spectra were introduced to further

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confirmthe FT-IR assignments. The XPS of nanoparticles showed three predominant

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peaks at 285 eV, 400 eV 532 eV (Figure S4), which were attributed to three elements

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namely C, N and O, respectively. Their relative contents were ca. 78.31%, 1.77%, and

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19.7% based on the calculation of the integral area. The results further demonstrated

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the carbon-rich nature. The high-resolution C1s XPS spectra exhibit characteristic

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peaks of C=C, C-N, C-O, C=O at 284.5, 285.5, 286.4, and 288.6 eV25, respectively

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(Figure 2B). The N1s high-resolution spectra (Figure 2C) around 399.3, 400.0 eV were

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assigned to the amino N and pyrrolic-like N, respectively34. The O1s XPS spectra

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(Figure 2D) could be decomposed into peaks at ca. 531.4, 532.3 and 533.2 eV,

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indicating the presence of *O=C-O, C-O, and O=C-O*28. Thus, these findings

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provided evidence that the hydroxyl, amino, and carboxyl exist on the surface of

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nanoparticles of the baked lamb. All of these results indicated that the fluorescent

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nanoparticles extracted from baked lamb had similar characteristics of CDs that were

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synthesized artificially with small molecule chemicals as raw material35-37. Therefore,



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these nanoparticles might be the CD-like nanostructures produced during baking

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process at high temperature. From this point of view, these fluorescent nanoparticles

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might be a class of foodborne CDs yet have not previously been found.

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Fluorescent stability. Fluorescence intensity of lamb-borne CDs under various

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pH, high ionic strength and continuous illumination were investigated in order to

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evaluate their stability. The CDs were observed to have relatively stable emission at

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broad pH ranges of 2-10 (Figure S5). The fluorescence intensity was almost

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unaffected by pH when the pH range was 4-9. A decrease of fluorescence intensity

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noticed only when pH was higher than 10 or lower than 4. It was notable that the

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fluorescence intensity decreased only about 30% under extreme pH conditions.

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Higher ionic strength (up to 10 g·L-1 NaCl) had a negligible influence on the

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fluorescence intensity (Figure S6). The fluorescence intensity dropped by about 0.1%

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in 1800 s under continuous UV radiation (370 nm), indicating the good photostability

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of CDs (Figure S7). Thus, the CDs from baked lamb were relatively stable to pH, high

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ionic strength and photo-bleaching.

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Fluorescence quenching of CDs by H2O2 and Fe2+. The functional groups on

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the surface such as hydroxyl group are reductive, under strong oxidizing conditions.

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Such reductive groups may be oxidized and thus lead to florescence quenching. The

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effect of H2O2 and Fe2+ on CDs fluorescence was investigated in order to prove this

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concept. As shown in Figure 3A, the fluorescence intensity of the CDs was almost

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unaffected by either H2O2 or Fe2+, indicating that the presence of H2O2 or Fe2+ has

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little effect on CDs. Furthermore, common metal ions showed almost no effect (less


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than 20%) on fluorescence intensity of CDs (Figure S8). However, an obvious

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fluorescence quenching was observed when H2O2 and Fe2+ were added

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simultaneously. The highly reactive hydroxyl radical product from the reactions

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between Fe2+ and H2O2 may oxidize the surface groups and thus lead to effective

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fluorescence quenching. In order to prove the involvement of hydroxyl radicals on the

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oxidation of the CDs, the hydroxyl radicals concentrations were detected by ESR and

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changes of surface groups on CDs surface were also investigated by using FT-IR and

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XPS after hydroxyl radicals attack. The effect of CDs on hydroxyl radical scavenging

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is shown in Figure 3B.The strong ESR signals indicated the formation of DMPO/·OH

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adduct, while the presence of CDs significantly decreased the ESR signal.

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Approximately 73% decrease was achieved with 1.95 mg·mL-1 CDs. As shown in

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Figure 3C, compared to the original CDs (Figure 2A), the intensity of the peaks around

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1404 and 3300 cm-1, representing the symmetric stretching vibration of COOH and –

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OH respectively, increased. Meanwhile, the disappearance of the peak at 2963 cm-1

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revealed the oxidation of methylene. Furthermore, the oxidation of CDs was also

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confirmed by XPS. The spectral area of –C=O and C–O from C1s high resolution

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spectra increased from 5.9% and 6.9% to 6.6% and 10.7% after oxidation treatment

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(Figure 3D) respectively, implying a corresponding increase in the content of –C=O

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and C–O in CDs. Therefore, these results further verified the conjecture that the

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fluorescence quenching of CDs resulted from surface oxidation of CDs by hydroxyl

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radicals.



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Application of CDs for glucose detection. The abundant presence of functional

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groups, excellent stability and hydroxyl radical induced fluorescence quenching

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demonstrated potential sensing applications of CDs from baked lamb. Herein, glucose

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was chosen as an example to demonstrate the sensor ability of those CDs. Glucose

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oxidase could quantitatively and specifically convert glucose to H2O2. In the presence

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of Fe2+, hydroxyl radicals were formed through the reaction between the Fe2+ and

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H2O2. As described above, hydroxyl radicals could oxidize the functional groups on

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CDs surface and thus quenched the fluorescence. Since the concentration of hydroxyl

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radicals was related to the glucose, quantification detection of glucose could be

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achieved by detecting the fluorescence intensity of CDs. Obviously, the glucose

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detection procedure involves two reactions: one being H2O2 generation process which

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was catalyzed by glucose oxidase, one being the hydroxyl radicals formation process

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that quenches the fluorescence of CDs after introduce Fe2+. Therefore, conditions for

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hydroxyl radical generation reaction and enzymatic reaction should be optimized

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separately in order to improve the sensitivity of the proposed biosensor for glucose

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detection.

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In order to ascertain the optimum conditions for hydroxyl radical generation

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reaction, the pH of buffer solution, the concentration of Fe2+ and incubation time were

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varied to investigate their effects on the fluorescence intensity of the CDs. The pH

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values adopted in this study were 2, 3, 4 and 5 (BR Buffer, 0.02 mmol·L-1), while the

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concentration of Fe2+ (0.45 mmol·L-1) and H2O2 (0.45mmol·L-1) remained constant. As

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shown in Figure 4A, the fluorescence intensity reached its minimum value at pH 3.0,


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therefore, the buffer of pH 3.0 was used in this experiment. Subsequently, the influence

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of the concentration of Fe2+ on fluorescence-quenched efficiency was investigated at

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pH 3.0 in the presence of H2O2 (0.45 mmol·L-1) (Figure 4B). The results indicated the

297

maximum fluorescence-quenched efficiency which

298

concentration of Fe2+ was 0.8 mmol·L-1. Hence, 0.8 mmol·L-1 was selected as the

299

optimum concentration for Fe2+. According to the results of Figure 4A and B, the

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fluorescence intensity decreased dramatically in the initial and remained unchanged

301

after 5 min, thus 5 min was chosen as appropriate reaction time for the quenching

302

process. Under the optimal experimental conditions, the fluorescence intensity of CDs

303

decreased as the amount of H2O2 increased (Figure 4C). Fluorescence quenching

304

efficiency of (F0-F)/F0 was introduced to assess quenching ability of H2O2, where F0

305

and F represent the fluorescence intensity before and after the addition of certain

306

amount of H2O2, respectively. A good linear relationship between (F0-F)/F0 and the

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concentration of H2O2 in the range of 0-100 µmol·L-1 was obtained by plotting

308

(F0-F)/F0 versus the concentration of H2O2 (Figure 4D), (F0-F)/F0=0.0054 C,

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R2=0.9936. These results confirmed the quantitative relationship between H2O2

310

concentration and fluorescence quenching efficiency.

was achieved when the

311

The optimum pH for glucose oxidase is 5.7, while the optimum pH for hydroxyl

312

radical generation reaction was 3. In order to obtain maximum sensitivity for the

313

fluorescent detection, the pH value was adjusted to 3 with 1 mol·L-1HCl after the

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enzyme reaction. For enzymatic reaction, the effects of glucose oxidases concentration

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and reaction time were optimized. The effort on the optimal glucose oxidase



316

concentrations was carried out by varying the glucose oxidase concentrations (1.0, 3.0,

317

5.0 and 10.0 U·mL-1) at a constant incubation time of 20 min. The fluorescence

318

intensity was independent to glucose oxidase which revealed that 1.0 U·mL-1 glucose

319

oxidase was sufficient to convert glucose to H2O2 (Figure 5A). As shown in Figure 5B,

320

the maximum quenching was observed after 20 min reaction, after which no apparent

321

change was observed. Therefore, 20 min was used as the incubation time for enzymatic

322

reaction.

323

Under optimum experimental conditions, the analytical performance of system

324

for quantitative detection of glucose was evaluated. As shown in Figure 5C, the

325

decrease of fluorescence intensity was dependent on glucose concentration.The

326

fluorescence quenching efficiency and the concentration of glucose exhibited a good

327

liner relationship in the range of 10 to 300 µmol·L-1with the following equation:

328

(F0-F)/F0=0.03338+0.0014 C (R2=0.9941) (Figure 5D). The detection limit was as low

329

as 2.9 µmol·L-1. The result was comparable to some reported methods for glucose

330

detection (Table. S2).

331

Selectivity of the biosensor. Selectivity of the developed biosensor is of great

332

importance for the detection of glucose. Thus, the effects of small biological molecules

333

(D-fructose, α-lactose, sucrose, chitosan, D-galactose, D-mannose, D-maltose,

334

D-xylose and dextran) on fluorescence quenching were investigated (Figure S9). The

335

result indicated that interfering species listed above were negligible compared with that

336

of glucose. The high selectivity could be attributed to the high affinity of glucose

337

oxidase for glucose. This result proved that the biosensor based on CDs from baked


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lamb could selectively detect the concentration of glucose and offer credible results

339

even when the interfering species reached high concentration.

340

Determination of glucose in real samples. To evaluate the practical application

341

of the developed method, the biosensor developed with CDs from baked lamb was

342

employed to determine glucose level in a real sample (apple juice). No sample

343

pretreatment was needed except for the water dilution with the real juice sample. The

344

results showed that the glucose levels of the beverage sample was 272 ± 14 mmol·L-1,

345

very similar to that of 288±12 mmol·L-1 obtained by ion chromatography.

346

In summary, this work demonstrated the presence of the CD-like fluorescence

347

nanoparticles in baked lamb. These nanoparticles emitted strong blue light under

348

ultraviolet radiation and were relatively stable to pH, high ionic strength and

349

photo-bleaching. The abundant reductive groups on the surface of such nanoparticles

350

could be oxidized by hydroxyl radicals. In addition, they could serve as a fluorescence

351

sensor for glucose detection. Our findings about the presence of CDs in baked lamb

352

provide new knowledge about nanoparticles in food which expands their potential

353

applications.

354

Acknowledgement

355

This work was supported by the National Natural Science Foundation of China

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(31601389)

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(2017YFD0400100, 2016YFD0400404).

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Supporting Information Available: [Photographs of solutions of baked and fresh

359

lamb under visible light and 365 nm UV light; X-ray diffraction (XRD) pattern of the

and

the

National

Key

Research

and


Development

Project


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fluorescent nanoparticles from baked lamb; Plots of integrated fluorescence intensity

361

of fluorescent nanoparticles and quinine sulfate as a function of optical absorbance at

362

370 nm; XPS survey of fluorescent nanoparticles; pH and NaCl effects on fluorescent

363

intensity of fluorescent nanoparticles; Time-course plot of fluorescence intensity of

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fluorescent nanoparticles during continuous irradiation; Effects of metal ions on

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fluorescence intensity of fluorescent nanoparticles; Selectivity of the detection system;

366

Parameters for QY calculation; Comparisons of different material for glucose assay]

367

Conflict of interest

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The authors declare that they have no conflicts of interest.

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References

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Figure captions:

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Figure 1 TEM image of fluorescent nanoparticles from baked lamb, the inset image

475

illustrates the lattice of the nanoparticle (A). Size distribution histogram and Gauss

476

fitting of particle size (B). UV–vis absorption and fluorescent emission spectra (C)

477

and the fluorescence decay profile of nanoparticles (D).

478

Figure2 FT-IR spectra of fluorescent nanoparticles (A). High-resolution XPS of C1s

479

(B), N1s (C) and O1s (D) of fluorescent nanoparticles.

480

Figure3 FL spectra of CDs solution (A). The black curve represents CDs solution, the

481

red curve represents CDs containing Fe2+, the blue curve represents CDs solution

482

containing H2O2, the pink curve represents CDs solution containing Fe2+ and H2O2, the

483

concentration of Fe2+ and H2O2 were 0.45mmol L-1, respectively. The scavenging

484

capacity of CDs against hydroxyl radicals (·OH) determined by ESR using

485

DMPO/·OH system without CDs as a control (B). FT-IR (C) and high-resolution XPS

486

spectra of C1s (D) of CDs after oxidized by hydroxyl radicals.

487

Figure4 The effects of pH (A) and Fe2+ concentration (B) on fluorescence quenching of

488

CDs in the presence of H2O2 and Fe2+. Fluorescence emission spectra of CDs at

489

different concentration of H2O2 solution in the presence of Fe2+ at room temperature(C).

490

Fluorescence quenching efficiency of CDs at various concentration of H2O2 (D). The

491

concentration of CDs was 0.1 mg·mL-1, the concentration of Fe2+ was 0.8 mmol·L-1,



492

the incubation time was 5 min, pH value was 3.0 and excitation wavelength was 370

493

nm. F0 and F are the FL intensity of CDs in the absence and presence of both H2O2 and

494

Fe2+, respectively. The inset shows a linear relationship with respect to fluorescence

495

quenching efficiency and H2O2 concentration.

496

Figure5 Effects of various concentration of glucose oxidase (A) and different

497

incubation time (B) of enzyme reaction on fluorescence quenching efficiency of CDs

498

in the presence of H2O2 and Fe2+. Fluorescence quenching efficiency of CDs for

499

various concentration of glucose (C), and linear relationship with respect to

500

fluorescence quenching efficiency and glucose level (D). The concentration of CDs is

501

0.1 mg·mL-1. The excitation wavelength is 370 nm. The incubation time of Fenton

502

reaction is 5 min. F0 and F are the FL intensity of CDs in the absence and presence of

503

both H2O2 and Fe2+, respectively.

504


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Figure 5

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Graphic for table of contents

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Presence of fluorescent carbon nanoparticles in baked lamb: Their

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