Preparation And Immobilization Of Fluorescent Gold Nanoclusters

Fluorescent gold nanoclusters?FGC?have been recognized as one of the most intriguing nanomaterials in recent years owing to ultra-small size,good photostability,and low biotoxicity.FGC are usually synthesized by reducing Au?III?under the protection of suitable ligands,such as protein,peptide and thiol-containing small molecule.Among various protecting ligands for FGC synthesis,protein is likely the most ideal one since it directly reduces Au?III?to FGC without requiring additional reducing agents and protects them from aggregation or further growth into non-fluorescent larger nanoparticles.Furthermore,most protein ligands maintain their biological activity after FGC synthesis,endowing FGC with extra functionality such as biocatalysis and antimicrobial activity.To date,many proteins have been used for FGC preparation,including bovine serum albumin?BSA?,transferrin,aprotinin and esterase,etc.However,it remains a challenge to apply the FGC to practical applications since most protein ligands are extracted from animals or microorganism,which results in a lack of availability,high costs,and tedious separation and purification processes.An effective way to solve these issues would be the use of low-cost,sustainable,and abundant plant proteins for FGC preparation.FGC with unique properties of fluorescence,catalytic and magnetism can be used in fields of chemistry,biology,and materials science.But FGC are often in aqueous solutions,which greatly limits its applications.Therefore there are many studies dedicated to immobilizing FGC.Recently,several studies have demonstrated FGC-based solid sensors for metal ions through the immobilization of FGC onto filter paper,glass,polyelectrolytes,electrospun membrane,and silica.However,FGC fluorescence is easily affected by these immobilization processes.Furthermore,the matrix materials may block the interaction between FGC and metal ions and thus delay the FGC fluorescence response.Therefore,it is necessary to develop an immobilization method that will not affect the interaction between FGC and metal ions.In addition,some research groups have fix FGC for preparing light-emitting diodes?LEDs?and used as a recyclable mimic enzyme.However,choosing more suitable FGC,the more environmentally friendly matrix and easier method are still worth studying.In this paper,FGC was used as the research object to study how to prepare FGC with easily extracted and cheap plant proteins and how to use the film-forming properties of the ligand to achieve the immobilization of Z-FGC.Later the electrostatic interaction between the ligand and the inorganic salt was used to achieve the immobilization of GSH-FGC.Our research work is as follows:We used zein,a low-cost,sustainable and abundant plant protein,for preparing FGC.Zein is the main protein in corn,which is one of the three major food crops in the world,and can be easily extracted from the by-products of corn starch and bioethanol manufacture.For the synthesis of FGC,the pH value,reaction time,reaction temperature and reagent concentration play a decisive role in whether FGC is successful prepared and have high quantum yield.Therefore,we studied from these four aspects and successfully synthesized fluorescent gold nanoclusters protected by zein?Z-FGC?exhibiting dual emission at 442 and 655 nm under excitation at 365 nm.In addition zein is soluble in 60-90%ethanol and alkaline solutions and is insoluble in acidic solutions.That is why for Z-FGC,the time-consuming dialysis process can be replaced by simply mixing the Z-FGC solution with acidic for a few minutes.And upon the introduction of Ag⁺/Hg²⁺ions,the Z-FGC emission at655 nm can be significantly enhanced/quenched while leaving the fluorescence at 442 nm almost unchanged.Accordingly,Z-FGC can be employed as fluorescent ratiometric probes for the detection of Ag⁺/Hg²⁺ions and the reasons why Ag⁺enhanced fluorescence and Hg²⁺reduced fluorescence were discussed.The Z-FGC probes function well under a broad range of pH?pH 6-10?and temperatures?20-60°C?and can thus be applied to determine Ag⁺/Hg²⁺ions in tap water.Furthermore,the self-assembled film-forming property of zein is preserved after the FGC were synthesized,which allows for the facile preparation of Z-FGC film for real-time,visual sensing of Ag⁺/Hg²⁺ions.In the second part of the work,the immobilization of glutathione-protected gold nanoclusters?GSH-FGC?was mainly addressed.However for the immobilization of FGC,the stability of GSH-FGC in the salt solution of high concentration was tested firstly and discussed the relationship between the radius and charge of the cation and the fluorescence intensity of GSH-FGC.According to above the synthesized GSH-FGC was placed in a saturated solution of NaCl to prepare a mixed crystal with high quantum yield and high stability.Subsequently,a stable,green and economical LED was designed using a hybrid crystal emitting orange fluorescence.The prepared LED will become a very promising material due to its light stability,thermal stability and high luminous efficiency.This also provides a new method for preparing other LEDs with various metal nanoclusters in this way.Then we fixed GSH-FGC in calcium fluoride,which could improve its catalytic activity and make it reusable.This allows gold nanoclusters to be better applied in the field of catalysis.