Aggregation-induced emission from gold nanoclusters for u as a
luminescence-enhanced ions
Bingzhi Li,Xi a r t i c l e i n f o Article history:
Received 15December 2015Accepted 3January 2016
Available online 4January 2016Keywords:
Gold nanoclusters
Aggregation-induced emission Luminescence enhancement Silver ions
音乐剧金沙
Charge transfer
a b s t r a c t
Several rearch have reported that silver ions (Ag +)could enhance the photoluminescence of some kinds of gold nanoclusters (AuNCs),and redox reaction involved mechanisms were recognized as the main rea-son to cau such phenomenon.However,in this work,we found that Ag +could enhance the lumines-cence of aggregation-induced emission gold nanoclusters (AIE-AuNCs)without valence state change.Upon addition of Ag +,the luminescence of AIE-AuNCs enhanced instantly by 7.2times with a red-shift of emission peak and a complete restoration of luminescence features was obrved when Ag +was removed.A cost-effective,rapid-respon,highly nsitive and lective method to detect trace amount of Ag +has thereby been established using AIE-AuNCs as a nanonsor.This analytical method exhibited a linear range of 0.5nM–20l M with a limit of detection of 0.2nM and it showed great promi for Ag +monitoring in environmental water.
Ó2016Elvier Inc.All rights rerved.
1.Introduction
Gold nanoclusters (AuNCs)are a type of nanomaterials smaller than 2nm compod of a gold core and a layer of protecting ligands [1].With a sub-nanometer size which is comparable to the Fermi wave length of electrons,AuNCs showing molecule-like properties in photoluminescent features [2].Since the confor-mation of Au core and the type of protecting ligands both had large influence on the properties
of AuNCs,great efforts have been made to explore synthetic strategies [3].The conventional synthetic route for AuNCs protected
by thiol containing molecules (including small peptides)is two-pot Brust–Schiffrin method and its modified versions.This strategy including a reduction process to form Au(I)with the help of reduction agents and a nucleation process to fabricate AuNCs.Though different ligands and reduction agents were ud,the structure of this conventional AuNCs is short
dx.doi/10.1016/j.jcis.2016.01.002
0021-9797/Ó2016Elvier Inc.All rights rerved.
⇑Corresponding author at:Nanjing Medical University,No.818Tianyuan East Road,Nanjing 211166,Jiangsu,PR China.
E-mail address:xueminzhou001_ (X.Zhou).
Au(I)–thiolate motifs populated on the surface of Au(0)core[4]. With a low Au(I)/Au(0)and thiol–Au ratios,conventional AuNCs is easy to be oxidized by oxygen so the application is lim-ited[1,5].
Recently,bad on aggregation-induced emission(AIE)mecha-nism,a bright AuNCs has been one-pot synthesized with a glu-tathione(GSH)protected Au(0)@Au(I)–GSH core–shell structure [6].Different from conventional AuNCs with short Au(I)–thiolate motifs,AIE-AuNCs had long oligomeric Au(I)–GSH complex shell aggregated around the Au(0)core.Becau of the oligomeric shell, AIE-AuNCs had a high Au(I)/Au(0)and thiol–Au ratios which were both much higher than conventional AuNCs[7–10].Derived from this structure,AIE-AuNCs had good stability to resist oxidation as well as high quantum yield(QY,$15%)and it has been employed as imaging agent[11],photonsitizer[12],radiotherapy nsitizer [13],etc.
The outstanding physical and chemical properties of silver have triggered its wide applications in jewelry,coins,medical products, imaging industries,electrical and electronic equipment[14,15], while the silver ions(Ag+)pollution caud by irresponsibly dis-posing of Ag-containing products has become a major environmen-tal issue.As a heavy metal ion with the ability to inactivate sulphydryl e
nzymes,Ag+is highly toxic tofish and aquatic microorganisms with a concentration higher than1.6nM[16].As to human,over intake of silver can cau argyria which is a dia result from silver deposition.Besides,Ag+have extensive effects to multiple systems including immune system,nervous system,and digestive system,etc[15].Various instruments have been applied to detect Ag+,including inductively coupled plasma-mass spec-trometry(ICP-MS)[17,18],atomic absorption spectroscopy(AAS) [19],inductively coupled plasma optical emission spectrometry (ICP-OES)[20],but the expensive equipment can’t be applied in real-time and on-site monitoring of Ag+.To solve this problem, many nanomaterials bad nsors have been developed,such as electrochemistry nsors[21,22]and optic nsors[23,24],but most of them are not cost-effective enough becau the u of rel-atively high-priced components and few of them can meet the requirement of nsing Ag+less than1.6nM.Hence,quick quantifi-cation of trace amount of Ag+in environment samples with reason-able cost is still a challenge.
Several rearch have reported that Ag+could influence the photoluminescence of conventional AuNCs.However,due to the difference in the ligands and core conformation,different mecha-nisms were propod.Jin’s group propod an oxidation state related principle to explain thefluorescent enhancement of GSH protected AuNCs[25].While Chang’s group reported that the pho-toluminescen
ce of6-MA protected AuNCs could be quenched by Ag+due to the d10–d10interaction between Ag and Au[26]. Besides,AuNCs capped by protein and cytosine was reported to be ud as turn-on probes for Ag+detection through mechanisms related to the reduction of Ag+[27,28].Among all the propod mechanisms,most of them pointed out that the valence state change was the key factor led to photoluminescence enhancement of AuNCs.
In this rearch,wefirstly found that Ag+can dramatically increa the luminescence of AIE-AuNCs without redox reaction. Tests results showed that the mechanisms of previously reported fluorescence enhancement caud by Ag+didn’tfit prent phe-nomenon,so a possible mechanism was discusd.Furthermore, we demonstrated that AIE-AuNCs can be ud as a promising Ag+ nanonsor with a limit of detection of0.2nM.This nanonsor showed high lectivity towards Ag+over other common metal ions and heavy metal ions including Ca2+,K+,Mg2+,Na+,Zn2+, Cu2+,Fe2+,Fe3+,Cd2+,Pb2+,Hg2+.This propod method was cost-effective and rapid-respon,so it was easy to be performed in real-time and on-site detection.2.Experimental ction
2.1.Materials
Hydrogen tetrachloroaurate trihydrate(HAuCl4),nitric acid (HNO3)were purchad from Shanghai Re
agent Co.(Shanghai, China).L-Glutathione in the reduced form(GSH)was obtained from Xiya Reagent(Shandong,China).Silver nitrate(AgNO3),sodium nitrate(NaNO3)and other reagents were purchad from Sino-pharm Chemical Reagent(Shanghai,China).All of the reagents were of analytical grade and ud without further treatment.Aqu-eous solutions were prepared with ultrapure water(18.2M X cm) produced by a Millipore water purification system(MA,USA).
2.2.Apparatus
Photoluminescent(PL)emission spectra were recorded on a Hitachi F-4600(Japan)fluorescence instrument and excitation light at400nm was ud to excite the samples.The lamp was kept on for1h prior to measurement in order to reduce thefluctuation of emission intensity.Shimadzu UV-2450(Japan)spectrophotome-ter was ud for UV–vis absorption measurements.X-ray photo-electron spectroscopy(XPS)spectra were collected in a PHI Quantera II spectrometer(Japan)and samples were freeze dried before XPS measurements.The transmission electron microscopic (TEM)images were acquired on a JEOL JEM-2100(HR)transmis-sion electron microscope(Japan).
2.3.Synthesis and purification of AIE-AuNCs
AIE-AuNCs were prepared as follows[6].Freshly prepared aque-ous solutions of HAuCl4(25mM,0.4
mL)were mixed with GSH solution(25mM,0.6mL)and4mL of ultrapure water under vigor-ous stir at ambient temperature and stirred until the solution turned colorless.Then the solution was heated to70°C under gen-tle stir for24h.The light yellow solution was dialyzed using a 3000Da MWCO dialysis bag against deionized water for24h with
a water change every4h.
2.4.pH adjusting
1mM HNO3and1mM NaOH were dropped to ultrapure water to acquire solutions with pH values ranged from3.0to6.5and the ionic strength of them were unified by adding NaNO3solution[28]. The obtained solutions were referred as pH adjusted solutions hereafter.白体
Solutions were all prepared and diluted by pH adjusted solu-tions with desired pH values for analytical condition optimization. HNO3and NaOH were directly added to adjust the pH values of the real samples[21].
指桑骂槐的近义词2.5.The removal of Ag+from Ag@AIE-AuNCs
Ag@AIE-AuNCs solution was prepared by mixing100l L of Ag+ (100l M)and50l L of AIE-AuNCs soluti
on and then100l L of GSH, cysteine(Cys)and ethylene diamine tetraacetic acid(EDTA)solu-tion with concentration ranged from20l M to600l M were intro-duced into it.Then emission spectra were recorded after conds of shaking.
2.6.Sensitivity and lectivity measurement花螺怎么做
Ag+solution of various concentrations were obtained by rial dilution of a stock solution(10mM)with pH adjusted solution (pH5.0).50l L of the as-prepared AIE-AuNCs solution was mixed
B.Li et al./Journal of Colloid and Interface Science467(2016)90–9691
with450l L of pH adjusted solution(pH5.0),followed by the addi-tion of100l L of Ag+solution with various concentrations.
100l M solution of other common cations(Ca2+,K+,Mg2+,Na+, Zn2+,Cu2+,Fe2+,Fe3+,Cd2+,Pb2+,Hg2+)were prepared using pH adjusted solution(pH5.0).50l L of the as-prepared AIE-AuNCs solution was mixed with450l L of pH adjusted solution(pH5.0) and metal ion solution(100l M,100l L).
2.7.Preparation and analysis of real samples
The lake water samples were collected from Xuanwu Lake in Nanjing and the tap water were directly obtained in our laboratory. Water samples werefiltered through a0.22l m membrane before the analysis.Then pH values were adjusted to5.0according to the was deconvoluted into Au(I)and Au(0)components with binding energies of84.3and83.7eV and Au(I)account for$73%of all Au atoms,which was clo to former reported($75%)[6].TEM images showed that the size of the as-prepared AIE-AuNCs was$1.8nm (Fig.S1).All the characteristics were consistent with previous reports,which confirmed the successful preparation of AIE-AuNCs.
3.2.Ag+induced luminescence enhancement
Surprisingly,the addition of Ag+caud an impressive lumines-cence enhancement of AIE-AuNCs.As provided in Fig.1b,the emis-sion peaks gradually red-shifted from610nm to630nm with the addition of0–100l M of Ag+.Meanwhile,a7.2-fold enhancement
absorption spectrum(blue solid line)and luminescent spectrum(red solid line)of AIE-AuNCs.(b)PL spectra of AIE-AuNCs after addition of
600nM,1l M,30l M,60l M,80l M,100l M AgNO3solution.The int is the digital photos of corresponding samples under UV lamp Ag@AIE-AuNCs with addition of different concentration of K
Br.(d)UV–vis absorption spectra of AIE-AuNCs with addition of0(black solid line),1l solid line)of AgNO3solution.Int is the digital photos of corresponding samples all emission spectra were recorded under the excitation
the references to color in thisfigure legend,the reader is referred to the web version of this article.)
92 B.Li et al./Journal of Colloid and Interface Science467(2016)90–96
added to the dialyzed luminescence-enhanced NCs,the lumines-cent features of it could be totally restored to AIE-AuNCs (Fig.1c),which meant the luminescence change was reversible and the Ag+was removable from the NCs.
UV–vis spectroscopy was ud to provide electronic structure and atomic composition information of NCs[30].Upon addition of more Ag+,the whole absorption profile shifted up without other changes in the spectra(Fig.1d)and the color of corresponding samples became deeper under visible light(Fig.1d int).The abnce of surface plasmon resonance(SPR)peak at$520nm indi-cated that no spherical nanocrystals larger than2nm was formed [34]which were consistent to TEM results.The retainment of char-acteristic absorption of AIE-AuNCs suggested that the aggregation-bad structure was possibly remained.
3.3.XPS analysis of AIE-AuNCs before and after Ag+addition
XPS was utilized to confirm the valence states of NCs.The luminescence-enhanced NCs were dialyzed in a dark place using a3KDa MWCO dialysis bag against deionized water for48h with a water change every4h to remove unbonded Ag+.The XPS spec-trum of AIE-AuNCs was also measured as a comparison.As dis-played in Fig.S3,the full-scan spectra of luminescence-enhanced NCs(blue line)and AIE-AuNCs(black line)were put together. Peaks of Ag could be obrved which confirmed that Ag was com-bined with the AIE-AuNCs,so we u Ag@AIE-AuNCs to reprent the luminescence-enhanced products hereafter.The valence states of Au in the AIE-AuNCs and Ag@AIE-AuNCs were analyzed and high-resolution spectra of Au4f were provided in Fig.2a.No obvi-ous shift of binding energy was obrved and the two Au4f7/2 spectra were further deconvoluted into Au(I)and Au(0)compo-nents with binding energy of84.3and83.7eV[6].The mean value of Au(I)content in AIE-AuNCs and Ag@AIE-AuNCs were73.3%and 73.6%respectively according to triple measurements operated for each sample.Meanwhile the binding energy of Ag3d5/2at 367.8eV(Fig.2b)matched well with univalent Ag+in the pure 11-MUA-AgNCs[35,36].The results indicated that Ag+connected with the AIE-AuNCs to form Ag@AIE-AuNCs without change of valence states.
吃松子的好处
3.4.The interaction between AIE-AuNCs and Ag+
To get more information about the interaction of Ag+and AIE-AuNCs,GSH,cysteine(Cys)and ethylene diamine tetraacetic acid (EDTA)were introduced into Ag@AIE-AuNCs solutions.GSH and Cys are both thiol-containing while GSH is a tripeptide with molec-ular weight larger than Cys.EDTA is a common chelating agent with high affinity towards heavy metal ions.Equivalents were ud (relative to Ag+added)to reprent the amount of the three com-pounds.Luminescence change(I/I0)at630nm were plotted against the amount of three compounds in Fig.3a,and the red dash line meant the original luminescence of AIE-AuNCs under the same conditions(I/I0=1).It could be found that GSH(black solid line) and Cys(orange solid line)could both make the luminescence intensity recovered within1equiv while EDTA(blue solid line) caud no change even at6equiv.
AIE-AuNCs has a oligomeric Au(I)–GSH complex shell aggre-gated around the Au(0)core.It has been reported that the surface GSH had specific steric hindrance to prevent large-sized thiol molecules(such as GSH)from interacting with Ag(I)in the core of NCs,while small molecular Cys was able to inrt into the GSH layer and interact with Ag(I)in the core of NCs[37].According to aforementioned principle,Ag@AIE-AuNCs should have responded differently to GSH and Cys if Ag+was combined to the core of it.So the consistency result between them in Fig.3a implied that the
Ag+mainly anchor to the shell of Au(0)@AIE-AuNCs. Besides,the rapid-respon as well as the reversible features of luminescence enhancement also supported that the anchor point was in the shell of AIE-AuNCs.The reason was that the reaction between Ag+and core Au of NCs was time-consuming and always lead to irreversible structure change[26,32].
In the shell of AIE-AuNCs,there were two interactions can lead to removable Ag+-NCs connection.Thefirst is the affinity between Ag+and electron-rich groups(thiol,carboxyl,and amino groups) prent in the surface GSH[38],and the cond is the d10–d10 interaction of Ag(I)–Au(I)[39].Among all the interactions,thiol groups have the strongest affinity towards Ag+.However,there were no free thiols in AIE-AuNCs becau they have acted as link-ers to bridge Au(I)atoms[6,40].Besides,if thiols were the binding sites to combine Ag+,the competition effect between surface thiols and GSH/Cys should have prevented the latter from removing Ag+. Previous rearch indicated that the binding energy between Ag+ and electron-rich groups can reach$246kcal/mol[41],while the strength of d10–d10interaction is only7–11kcal/mol[42],so Ag+would preferentially combine to electron-rich groups.Our test results showed that EDTA was unable to remove Ag+from the NCs (Fig.3a blue solid line),which also supported that d10–d10inter-action wasn’t the major interaction to form Ag@AIE-AuNCs.
3.5.Possible mechanism for luminescence enhancement
Several groups have reported that direct addition of Ag+could lead to photoluminescence enhancement of AuNCs and a compar-ison between prent work and former related reports was listed
in Fig.2.(a)Au4f spectra of AIE-AuNCs and Ag@AIE-AuNCs.(b)Ag3d spectrum of Ag@AIE-AuNCs.
红枫的诗句
Table S1.Most of the former reports indicated that valence state change was the key point led tofluorescence/luminescence enhancement[25,27,28,32].Xie’s group has indicated that Ag+ could enhancefluorescence of AuNCs by connecting with thiol groups and bridging the small Au(I)–thiolate motifs without redox reaction[43].However,AIE-AuNCs have a long oligomeric Au(I)–GSH complex shell instead of Au(I)–thiolate motifs,and the Au(I) have already bridged by thiols.
Since the luminescence of AIE-AuNCs was generated from the aggregated oligomeric Au(I)–GSH complex shell[6,40],we pre-sumed that Ag+immobilized by surface electron-rich groups led to the change of charge transfer in the shell(Scheme1).As a proof of concept,oligomeric Au(I)–GSH complex was synthesized according to previous literature(Supplementary material).As indi-cated in Fig.3b oligomeric Au(I)–GSH complex aggregated in95%
a luminescence with a maximum emission at
consistent with former reports[6,44].When Ag+ the solution,a red-shift and enhancement of emis-
obrved.In our experiment,the highest enhance-transfer(LMCT)or ligand-to metal–metal charge transfer(LMMCT) of it[8,45–47].At prent we can’t direct characterize the accurate structure of Ag@AIE-AuNCs becau it was difficult to get the sin-gle crystal of it but more work will be done in the future to get a deep understanding of this mechanism.
3.6.Detection of Ag+bad on luminescence enhancement
To optimize the respon of AIE-AuNCs towards Ag+,pH effect was investigated by the change of relative intensity((I–I0)/I0,I r), and it showed the best luminescence respon at pH5.0(Fig.S5). Fig.4a prented the luminescence intensity at630nm against the concentration of Ag+.It could be found that there was a lumi-nescence decrea when the concentration of Ag+was higher than 100l M.A interpretation for this decrea was that when the bind-ing sites of the oligomeric shell was saturated,
雨夜钢琴combine to the Au(0)core of the NCs and affect
适应能力强
between Au(0)and Au(I)which has been reported
intensity(I/I0)of Ag@AIE-AuNCs samples after addition of different equivalents of EDTA(blue solid line),GSH(black solid line),and Cys luminescence of AIE-AuNCs.I/I0was calculated according to emission intensity at630nm.(b)PL spectra of Au(I)–GSH complex in and without(blue solid line)100l M Ag+.(For interpretation of the references to color in thisfigure legend,the reader is referred Scheme1.Schematic illustration of Ag+caud luminescence enhancement of AIE-AuNCs.
