materials Article Novel Graphene/In2O3 Nanocubes Preparation and Selective Electrochemical Detection for L-Lysine of Camellia nitidissima Chi 1, , 2, 1,3 1 4 Jinsheng Cheng * y , Sheng Zhong y, Weihong Wan , Xiaoyuan Chen , Ali Chen and Ying Cheng 3 1 Henry-Fork School of Food Sciences, Shaoguan University, Shaoguan 512005, China; [email protected] (W.W.); [email protected] (X.C.) 2 Shipai Branch, Dongguan Environmental Protection Bureau, Dongguan 523330, China; [email protected] 3 Foshan Qionglu Health Tech. Ltd., Foshan 528000, China; [email protected] 4 School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, China; [email protected] * Correspondence: [email protected] These authors contributed equally to this work. y Received: 2 March 2020; Accepted: 20 April 2020; Published: 24 April 2020 Abstract: In this work, novel graphene/In2O3 (GR/In2O3) nanocubes were prepared via one-pot solvothermal treatment, reduction reaction, and successive annealing technology at 600 ◦C step by step. Interestingly, In2O3 with featured cubic morphology was observed to grow on multi-layered graphene nanosheets, forming novel GR/In2O3 nanocubes. The resulting nanocomposites were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), etc. Further investigations demonstrated that a selective electrochemical sensor based on the prepared GR/In2O3 nanocubes can be achieved. By using the prepared GR/In2O3-based electrochemical sensor, the enantioselective and chem-selective performance, as well as the optimal conditions for L-Lysine detection in Camellia nitidissima Chi, were evaluated. The experimental results revealed that the GR/In2O3 nanocube-based electrochemical sensor showed good chiral recognition features for L-lysine in Camellia nitidissima Chi with a linear range of 0.23–30 µmol L 1, together with · − selectivity and anti-interference properties for other different amino acids in Camellia nitidissima Chi. Keywords: graphene/In2O3 cubes; L-Lysine; detection; Camellia nitidissima Chi 1. Introduction In the past years, graphene has gained various interests in different fields [1–5]. This novel 2D carbon-based nanomaterial possesses superior chemical, physical, and electrical properties [6–10]. A variety of intriguing graphene-based nanocomposites with different morphologies, for instance, nanoparticles [11], nanoshuttles [3], nanorods [12], nanofibers [13], nanosheets [14], metal organic frameworks (MOFs) [15], and quantum dots [16], etc., were prepared with different applications. Recently, some researchers studied various functional graphene nanocomposite-based electrochemical sensors for different applications. For example, our team developed a sodium dodecyl benzene sulphonate (SDBS) functionalized graphene nanosheet-based electrochemical biosensor, which showed excellent electrocatalytic performance toward the reduction of H2O2 with fast response, wide linear range, high sensitivity, and good stability [17]. Kang et al. employed thermally split graphene oxide, both of which exhibited similar excellent direct electrochemistry of glucose oxidase (GOD) [18]. However, there were few reports concerning the synthesis and application of transition-metal nanocubes/graphene nanocomposites. Materials 2020, 13, 1999; doi:10.3390/ma13081999 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 10 glucoseMaterials 2020oxidase, 13, 1999 (GOD) [18]. However, there were few reports concerning the synthesis 2and of 10 application of transition‐metal nanocubes/graphene nanocomposites. Camellia nitidissima Chi, a widely used plant in Southern China, is known as the “Giant Panda of Botany”Camellia [3]. In nitidissima2010, the MinistryChi, a widely of Health used of plantthe People’s in Southern Republic China, of China is known approved as the Camellia “Giant nitidissimaPanda of Chi Botany” as a [new3]. Inresource 2010, the of Ministryfood (2010 of Chinese Health ofMinistry the People’s of Health Republic Announcement of China approved No. 9). InvestigationsCamellia nitidissima revealedChi that as aCamellia new resource nitidissima of food Chi (2010contains Chinese rich amino Ministry acids, of vitamin Health AnnouncementC, gross sugar andNo. protein, 9). Investigations etc. Moreover, revealed the contents that Camellia of some nitidissima typical essentialChi contains amino rich acids amino (EAAs) acids, in cultivated vitamin C, Camelliagross sugar nitidissima and protein, Chi, for etc. example, Moreover, Leu, theThr, contents Val, and of Ile, some were typical higher essential than those amino in the acids Food (EAAs) and Agriculturein cultivated OrganizationCamellia nitidissima of the UnitedChi, Nations for example, (FAO)/World Leu, Thr, Health Val, andOrganization Ile, were (WHO) higher thanreference those contentsin the Food [19]. and Our Agriculture previous research Organization also showed of the that United by Nationsusing graphene (FAO)/World nanoshuttle Health matrix Organization‐based matrix(WHO)‐assisted reference laser contents desorption/ionization [19]. Our previous time of research flight mass also spectrometry showed that (MALDI by using‐TOF graphene MS), fifteennanoshuttle kinds matrix-basedof amino acid matrix-assisted ingredients, laserincluding desorption L‐lysine/ionization (L‐Lys), time L‐aspartic of flight massacid spectrometry(L‐Asp), L‐ threonine(MALDI-TOF (L‐Thr), MS), L‐serine fifteen (L kinds‐Ser), of L‐ aminoglutamic acid acid ingredients, (L‐Gly), etc., including can be detected L-lysine readily (L-Lys), in L-aspartic Camellia nitidissimaacid (L-Asp), Chi with L-threonine strong peaks (L-Thr), and L-serine good discrimination (L-Ser), L-glutamic [3]. Similar acid results (L-Gly), were etc., also can observed be detected in ourreadily other in work.Camellia By combining nitidissima withChi gas with chromatography strong peaks and‐mass good spectrometry discrimination (GC‐MS), [3]. Similarthe combined results stirwere bar also sorptive observed extraction in our (SBSE)/GC other work.‐MS By technology combining with with linear gas chromatography-mass graphene nanocomposite spectrometry coating can(GC-MS), detect seventeen the combined kinds stir of bar amino sorptive acids extraction in Camellia (SBSE) nitidissima/GC-MS Chi technology seeds, including with linear Ala, Gly, graphene Thr, Ser,nanocomposite Val, Leu, Ile, coatingCys, Pro, can Met, detect Asp, seventeen Phe, Glu, kindsLys, Tyr, of amino His, and acids Arg in [13].Camellia nitidissima Chi seeds, includingThe research Ala, Gly, on Thr, the Ser,homochirality Val, Leu, Ile, of Cys, basic Pro, biological Met, Asp, molecules Phe, Glu, such Lys, as Tyr, amino His, acids and Arg is still [13 ].an intriguingThe puzzle research in onmodern the homochirality sciences [20]. Research of basic biologicalon chiral recognition molecules and such chiral as amino selection acids of some is still kindsan intriguing of amino puzzle acids [21] in modern could provide sciences fundamental [20]. Research scientific on chiral thought recognition towards and chiral resolving selection the puzzle of some regardingkinds of amino some kinds acids [of21 homochirality] could provide and fundamental other important scientific applications. thought towards However, resolving until now, the puzzle few reportsregarding have some focused kinds on the of homochirality chiral recognition and of other a single important kind of applications.amino acid, for However, example, untilL‐Lys, now, in Camelliafew reports nitidissima have Chi. focused on the chiral recognition of a single kind of amino acid, for example, L-Lys,In this in Camellia study, nitidissimausing one‐Chi.pot solvothermal treatment, together with reduction and annealing treatmentsIn this in study, turn, usingwe successfully one-pot solvothermal prepared novel treatment, graphene/In together2O3 with (GR/In reduction2O3) nanocubes. and annealing The preparedtreatments nanocomposites in turn, we successfullywere characterized prepared by noveltransmission graphene electron/In2O3 (GRmicroscopy,/In2O3) nanocubes.scanning electronThe prepared microscopy, nanocomposites X‐ray diffraction were spectroscopy, characterized etc. by A transmission selective electrochemical electron microscopy, sensor for scanning L‐Lys inelectron Camellia microscopy, nitidissima Chi X-ray based diff ractionon GR/In spectroscopy,2O3 nanocubes etc. was A selective designed electrochemical and applied. In sensor a linear for range L-Lys ofin 0.23–30Camellia μmol nitidissima∙L−1, theChi GR/In based2O3 on‐based GR/In electrochemical2O3 nanocubes sensors was designed illustrated and applied.good chiral In a recognition linear range of 0.23–30 µmol L 1, the GR/In O -based electrochemical sensors illustrated good chiral recognition of L‐Lys in Camellia· − nitidissima Chi2 3with a strong selective signal. of L-Lys in Camellia nitidissima Chi with a strong selective signal. 2. Results and Discussion 2. Results and Discussion 2.1. Preparation and Characterization of GR/In2O3 Nanocubes 2.1. Preparation and Characterization of GR/In2O3 Nanocubes Until now, there have been few reports concerning the preparation and application of GR/In2O3 Until now, there have been few reports concerning the preparation and application of GR/In2O3 nanocubes, which would combine both superior electrical properties of graphene and In2O3 nanocubes, which would combine both superior electrical properties of graphene and In2O3 nanocubes. nanocubes. In this work, we prepared novel GR/In2O3