Study On The Synthesis And Properties Of Amino Acid Functionalized Graphene Quantum Dots

As a new member of graphene family,graphene quantum dot?GQD?has been discovered very recently as a kind of zero-dimensional nanomaterials.GQD overcomes the inherent stacking and accumulation of graphene due to the strong intermolecular forces between graphene sheets.The pronounced quantum confinement and edge effects of GQD confer them with superior photoluminescence.GQD possesses some excellent characteristics of both graphene and semiconductor quantum dot.Besides,GQD is superior in terms of bright and stable photoluminescence,high water solubility and low cytotoxicity in comparison to semiconductor quantum dots and graphene.All these advantages make GQD widely use in chemical sensing,bioimaging,medical therapeutics,energy-related applications and so on.However,the current GQD in literatures has a lot of disadvantages such as low fluorescence quantum yield,single function and poor functionality,and these shortcomings largely limit the wide applications of GQD.Therefore,it strongly requires to synthesize new GQD with specific function to satisfy various demands and further expand the application fields.In the present work,the synthesis of amino acid-functionalized GQD,and the influence of their structures on performance as well as the construction of hybrid composites were studied in detail.Twenty kinds of amino acid-functionalized GQD have been synthesized using 20 kinds of amino acid as functional reagents.The study on synthesis mechanism of GQD shows that the structure and spatial configuration significantly affect the formation of amino acidfunctionalized GQD.Amino acids with bigger steric hindrance tend to locate at the edge of sp2 clusters,while others with simple structures would partial dop into graphene planar.Optical properties study reveal that the introduction of nitrogen element in amino acid results in increasing fluorescence quantum yield of GQD obviously.Usually,nitrogen located at the edge of GQD can increase the electron density,which leads to the red-shifted of fluorescence spectral peak.But those in GQD internal planar will cause the blue-shift of fluorescence peak due to their strong electron withdrawing.Different composition and structures of surface functional groups of GQD will have significant effect on their fluorescence emission.Valine-functionalized GQD can be used as a fluorescence probe for sensing mercury ion with a higher sensitivity.It can be seen that the introduction of valine increases the fluorescence quenching effect of GQD towards mercury ion.The sensitivity and response time of Valine-functionalized GQD towards mercury ion were nearly 14 and 10 times than that of non-functionalized GQD prepared using citric acid only.Methionine-functionalized GQD shows a sensitive and liner response to pH in the environment,and a wide range pH fluorescence sensor based on above property was developed.When pH was in the range of 1 to 14,the fluorescence of Methionine-functionalized GQD increased with the increasing of medium pH value.And the developed sensor showed similar fluorescence response toward pH in other buffer systems.Compared with previous methods,the above two proposed methods possess improved analytical characteristics such as sensitivity,selectivity and response range.They have been successfully applied into mercury ion and pH value detection in water samples as well as monitor the pH-dependent fluorescence imaging inside of RBL-2H3 cells.The oxidation of malachite dye with H₂O₂ was used as a model reaction to examine the effect of structure of twenty kinds of amino acid-functionalized GQD on their catalytic performance in dye degradation efficiency.The results showed that the adsorption and catalysis of all 20 kinds of amino acid-functionalized GQD can significantly enhance the degradation of malachite.Among 20 kinds of amino acid-functionalized GQD,Histidine-functionalized GQD exhibited highest catalytic activity due to its nitrogen-rich imidazole ring,which can provide more alkaline active site.The reaction conditions were further optimized using Histidinefunctionalized GQD as catalyst.These optimal conditions were reaction temperature of 35?,malachite of 50 mg/L,catalyst of 0.5 mg/mL,and H₂O₂ of 15 mmol/L.Under these optimal conditions,the degradation efficiency of malachite reached 93% after 5 min.Besides,the proposed degradation process did not need any special light and other assisted conditions.The catalytic activity of Histidine-functionalized GQD has barely decreased after 10 recycle.The oxidation of tetramethylbenzidine with H₂O₂ catalyzed by Horse radish peroxidase?HRP?and the synthesis of L-menthy acetate by esterification of L-menthol and acetic anhydride catalyzed by Pseudomonas cepacia lipase?PCL?were used as model reactions,the performance of GQD in modulation of enzymatic reaction was examined.The results show that the size of modifiers,and the property and spatial configuration of their functional groups have great influence on enzyme performance.GO-GQD prepared using graphene oxide as precursor and Cysteine-functionlized GQD?Cys-GQD?exhibited contrasting effects on modulation of HRP performance.GO-GQD could dramatically improve the enzyme activity,while Cys-GQD showed a strong inhibition effect on HRP.Circular dichroism analysis pointed that the introduction of GO-GQD with smaller hydrophilic groups could increase the ordered enzyme structure,which could improve HRP activity.The blue oxidized tetramethylbenzidine can lead to an obvious fluorescence quenching of GQD.Based on the above fluorescence quenching effect and GO-GQD enhance HRP activity,a H₂O₂ sensor was developed using GO-GQD modified HRP as biocatalyst.The activity and stability of PCL modified with Cys-GQD were higher than that of PCL modified with graphene oxide.Under optimal conditions,the conversion of L-menthol and half-life of PCL were 97.3% and 224 h.The Cys-GQD modified PCL was recycled 10 times without substantial decrease in activity.How to combine GQD with other materials to realize their complementary advantages has become a research hotspot recently.Gold/GQD were prepared by one-step synthesis using GQD as reducing agent and stabilizing agent,and the effect of different structures on reduction property was studied.The results showed that the composition of functional groups on GQD had a great effect on the reducibility.Due to the fact that proline is the only natural amino acid containing a secondary amine within the pyrrolidine ring,the nitrogen atom in the secondary amine has a stronger coordination with gold ions due to its higher charge density.The structure makes Proline-functionalized GQD?Pro-GQD?shows highest reducibility.The formation of Gold/Pro-GQD can complete within 1 min.The resultant Gold/Pro-GQD has a clean surface loaded with Pro-GQD with a large conjugated system,the direct modification of gold nanoparticles with Pro-GQD realizes an ultrafast electron transfer.The electron transfer rate constant of Gold/Pro-GQD modified electrode in 5.0 mmol/L K4[Fe?CN?6] is 15.58±0.43 cm/s,which is 2.86 times higher than that of bare electrode.To further investigate the electrochemical of Gold/Pro-GQD,differential pulse voltammetry technique was used to detect acetaminophen.The obtained detection limit was 0.02 ?mol/L.Finally,a surface enhanced Raman scattering?SERS?-fluorescence dual mode probe was fabricated using bipyramid gold nanocrystal?BPGN?and GQD.First,an improved method was used for the synthesis of branched BPGN through the use of a small amount of sodium dodecylbenzen sulfonate?SDBS?additive.The introduction of SDBS allows reduction of hexadecyltrimethyammonium bromide to half.The synergistic effect of these two surfactants is beneficial to the formation of BPGN with excellent monodispersion and stable optical property,which can be used as an excellent SERS substrate.GQD with more excellent biocompatibility and fluorescence was used as fluorescence probe.BPGN reported SERS probe and GQD were assembled on two different layers in a silica sphere,which is a simpler and more controllable process compared to that of assembling agents in one layer.The probe provides a larger encoding capacity by simply change the SERS probe or fluorescence probe.Moreover,the dual probe was successfully applied to detect microcystin-LR using a sandwich immunoassay.