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Effects of linker molecules on the attachment and growth of gold nanoparticles

Electrochimica Acta 54(2009)5042–5047

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Electrochimica

Acta

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t

Effects of linker molecules on the attachment and growth of gold nanoparticles on indium tin oxide surfaces

Munetaka Oyama ?,1,Akiko Orimo,Khalid Nouneh 2

Department of Material Chemistry,Graduate School of Engineering,Kyoto University,Nishikyo-ku,Kyoto 615-8520,Japan

a r t i c l e i n f o Article history:

Received 31October 2008Received in revised form 22December 2008

Accepted 22December 2008Available online 12January 2009Keywords:

Gold nanoparticles Indium tin oxide Linker molecules Cysteamine

Nanostructural growth

a b s t r a c t

As a linker molecule to attach gold nanoparticles (AuNPs)on indium tin oxide (ITO)surfaces,cys-teamine was examined together with well-known 3-mercaptopropyltrimethoxysilane (MPTMS)and 3-aminopropyltrimethoxysilane (APTMS).Systematic comparisons of AuNPs’nanostructures formed on ITO were carried out after the room temperature treatment in an ethanol solution of linker molecules followed by the attachment of AuNPs using (i)one-step immersion into the 20-nm Au colloidal solu-tion or (ii)two-step immersion,i.e.,a seed-mediated growth treatment.Consequently,in case (i),it was found that APTMS was most effective for denser and homogeneous attachment of AuNPs on ITO surfaces with keeping the dispersion.With the MPTMS or cysteamine linkers in case (i),AuNPs aggregated on the ITO surfaces,and the attached amount of AuNPs were larger with the MPTMS linker.In contrast,in case (ii),nanostructural growth of AuNPs was possible with MPTMS or cysteamine using the seed-mediated growth,while the growth of AuNPs was signi?cantly suppressed with the APTMS linker.Because quite different results were obtained between APTMS and cysteamine,it was found that the attachment and nanostructural growth of AuNPs on the organic linker layers cannot be simply governed by the functional groups,–NH 2,on the outer surfaces.

1.Introduction

Metal nanoparticles (NPs)have been attracting active atten-tion as functional units for electroanalysis and bioelectroanalysis [1–3].If we would like to utilize metal NPs as unique devises in functional electrodes,normally the NPs need to be attached on the electrode surfaces.Focusing on the attachment of gold nanoparticles (AuNPs),which are typical functional nanometal that has biocompatibility and electrocatalytic functions [1,2],on the surfaces of indium tin oxide (ITO),which is a transparent and conducting material,the use of 3-mercaptopropyltrimethoxysilane (MPTMS)or 3-aminopropyltrimethoxysilane (APTMS)would be a well-known approach [4].With the MPTMS linker,the fabrication of Au nanoelectrode ensembles [5],and the ?ne tuning [6]and the manipulation of the AuNPs’growth on glass [7]have been reported.On the other hand,AuNPs attached with the APTMS linker have been used for further Au nanostructuring on the SiO 2[8]or glass [9]and the fabrication of colloidal Au modi?ed ITO electrode [10].

?Corresponding author.Tel.:+81753833074;fax:+81753833074.E-mail address:m.oyama@kx8.ecs.kyoto-u.ac.jp (M.Oyama).1

ISE member.2

On leave from Materials Science Units,CENM/CNESTEN,Rabat,Morocco.Before the linker attachment being applied to AuNPs,the modi-?cation of APTMS on ITO was reported for the purpose of attaching C 60on the ITO surface by Mirkin and coworkers [11].They also reported the use of cysteamine to attach C 60on Au surfaces [12],because the –SH terminal of cysteamine is suitable for the modi?cation on Au and the –NH 2for the bonding with deriva-tized C 60.Interestingly,recent reports have shown that cysteamine can be used also for the modi?cation of ITO through the –SH attachment on ITO surfaces,and that successful attachment of C 60and further fabrication of photofunctional ?lm are possible [13,14].

On the other hand,our group is proposing a different method-ology to attach AuNPs on the ITO surfaces [15–17].By applying a seed-mediated growth method,which was developed by Murphy and coworkers for synthesizing metal nanorods in aqueous solution [18,19],to the surface modi?cation,the crystal growth of AuNPs on the ITO surfaces could be performed from the Au nanoseed particles physically adsorbed on the ITO,with keeping adhesion contacts [15].As one of the advantages of this method in which no organic linker molecules were used,the charge transfer resistance (R ct )has signi?cantly decreased on the AuNPs directly attached ITO (AuNP/ITO)https://www.wendangku.net/doc/0e2548285.html,pared with the presence of a MPTMS layer,it was found that the R ct value could be reduced to ca.1/3on the AuNP/ITO electrode [16,17].

Because the R ct is expected to strongly depend on the iden-tity of linker molecules, e.g.,as the effects of AuNP-assembly

M.Oyama et al./Electrochimica Acta54(2009)5042–50475043

on the electrochemical behavior was investigated on11-amino-1-undecanethiol recently[20],cysteamine having shorter chain length would be a good candidate for the attachment of AuNPs for utilizing as the modi?ed electrodes.Thus,we started our work to explore the characteristics of cysteamine as a linker molecule to attach AuNPs on ITO surfaces.In addition,up to now,a selected linker tends to be used in each report,and the preparation method tends to be optimized for each purpose.So,systematic compar-isons of the attachment of AuNPs depending on linker molecules should be interesting and informative to know how different AuNPs’nanostructures are formed on ITO.Thus,in the present work,we compared the AuNPs’nanostructures formed on ITO using three linker molecules,i.e.,cysteamine,MPTMS and APTMS.

As the methods to prepare linker molecules’layer on the ITO sur-faces,sometime re?ux treatments in organic solvents were adopted as in the case of APTMS[11],but simple treatment using alcohol solutions was possible as in the cases of MPTMS[5,6]and APTMS [10].For cysteamine,the use of1.0mM ethanol solution has been normally used[12–14].Thus,in the present work,to take the advan-tage of simplicity,the treatment using the ethanol solutions was adopted for all the three linker molecules.

Furthermore,the attachment of AuNPs might depend on the capping reagents on AuNPs as well as the linker molecules.Thus, in the present work,we examined two kinds of AuNPs,i.e.,(i)com-mercially available20-nm Au colloidal solution,in which AuNPs are capped with tannic acid[21],and(ii)citrate-capped AuNPs whose size is ca.4nm[18].As for the former,we believe it important for general purpose to modify the ITO surfaces with the AuNPs having a de?ned size,and the latter is important in combining the growth treatment for further nanostructuring.

2.Experimental

2.1.Apparatus and materials

Scanning electron microscopic(SEM)images were obtained with a?eld-emission SEM instrument(JSM-7400F,JEOL,Japan). Electrochemical experiments were carried out with an EG&G M263A potentiostat(Princeton Applied Research,USA).

Glass plates on which ITO?lm was coated were purchased from CBC Optics Ltd.(sheet resistance;ca.50 /square,size 10mm×26mm).A piece of the ITO glass was cut into three pieces, and used after washing in acetone,ethanol and pure water with sonication and drying with nitrogen gas.

A20-nm Au colloid solution used was the product of Sigma. Cysteamine,MPTMS,APTMS,HAuCl4·3H2O and cetyltrimethylam-monium bromide(CTAB)were purchased from Aldrich.Other reagents were obtained from Wako Pure Chemicals.All aqueous solutions were prepared with ultra-pure water obtained from a water puri?cation system(Millipore WR600A,Yamato Co.,Japan) to a speci?c resistance>18M cm.

2.2.Preparation of AuNP-attached ITO substrates

For modifying ITO surfaces with linker molecules,a piece of ITO was immersed in5ml of the ethanol solution of linker molecules in a test tube for24h.For MPTMS and APTMS,5vol.%ethanol solu-tion was used,and,for cysteamine,1mM ethanol solution was used following the previous works[12–14].To maintain a constant tem-perature around a room temperature,a thermostated water bath at 28.0?C was used to keep the test tube.After the treatment,the ITO surface was washed well with pure water,and dried with nitrogen gas.

Then,as the?rst trial,the linker-modi?ed ITO was used for the one-step attachment with a20-nm Au colloid solution.Simply,the ITO was immersed in the Au colloid solution in a test tube for24h, and then washed with pure water,dried and used for the surface evaluation.In this case,before attaching AuNPs using a20-nm Au colloidal solution,we tried a heat treatment at40?C using a drying oven for1h to compare with the results without heat treatment for linker-modi?ed ITO surfaces.

As the other trial,the seed-mediated growth treatment was performed for linker-modi?ed ITO substrates.As the actual pro-cedure,a piece of linker-modi?ed ITO was immersed in a gold nanoseed solution for2h.The gold nanoseed solution contain-ing ca.4nm AuNPs was prepared by mixing0.5ml0.01M HAuCl4 with0.5ml0.01M trisodium citrate,20ml water and0.5ml 0.1M NaBH4fresh aqueous solution.Then,after thoroughly rinsed with pure water and dried with nitrogen gas,the substrate was immersed into a growth solution for24h.The growth solution was prepared by mixing20ml0.1M CTAB,0.5ml0.01M HAuCl4, 0.1ml0.1M ascorbic acid and0.1ml0.1M NaOH solutions.In this case,no heat treatment was performed before the growth treat-ment.

3.Results and discussion

3.1.Attachment of AuNPs after the treatment with20-nm Au

colloid solution

Fig.1shows the SEM images of the AuNP-attached ITO sur-faces prepare using the three linker molecules.Before immersing in the20-nm Au colloid solution,the linker-modi?ed ITO was used without heat treatment for this result.As shown in Fig.1,differ-ent results were obtained for AuNPs’attachment on the examined three linker https://www.wendangku.net/doc/0e2548285.html,ly,the aggregation of AuNPs was sig-ni?cant on the cysteamine and MPTMS linkers(Fig.1A and B),while AuNPs dispersed and attached on the surface of ITO on the APTMS layer(Fig.1C).Because each20-nm AuNPs could be recognized as a sphere of almost same size in the SEM images,the recognition of the aggregation and dispersion was easy as shown in Fig.1.How-ever,when we focus the AuNPs on the APTMS layer(Fig.1C),it could be recognized each NP was not independent,but connecting each other in many parts.As a difference between Fig.1A and B,the attached amount of AuNPs is higher on the MPTMS layer compared with that on the cysteamine layer.

Although the results of the SEM observation showed the local nanoscale images on the ITO surfaces,in order to obtain the information of macro-area,we observed the absorption spectra of the AuNP-attached ITO surfaces prepare using the three linker molecules without heat treatment.As expected from the SEM images in Fig.1,the absorption spectra in Fig.2shows some charac-teristics of the AuNP-attached ITO surfaces depending on the linker molecules.In the case with APTMS,the absorption(Fig.2c)was strongest among the three results,re?ecting the rich attachment of AuNPs as in Fig.1C.The appearance of two absorption peaks in Fig.2c should be the re?ection of the presence of the two states, i.e.,relatively independent AuNPs corresponding to the peak at the shorter wavelength(ca.530nm)and AuNPs connecting each other to the peak at the longer wavelength(ca.670nm).In the case with MPTMS,only the peak for gathered AuNPs at the longer wavelength is dominant as shown in Fig.2b,which is in accordance with the result of Fig.1B.The absorption with cysteamine(Fig.2a)was weak-est due to the smallest amount of AuNPs attached on the ITO surface (Fig.1A).

The above results showed that the attachment with the APTMS linker would be the best for the purpose of the attachment of 20-nm AuNPs keeping a dispersed state.On the other hand,the aggregation of AuNPs was signi?cant in the cases with MPTMS and cysteamine with the present simple treatment,i.e.,the immersion into the ethanol solution of linker molecules at room temper-

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Fig.1.Typical SEM images of AuNP-attached ITO surfaces prepared using three linker molecules,i.e.(A)cysteamine,(B)MPTMS and(C)APTMS,via the one-step immersion into the20-nm Au colloid solution.The linker-modi?ed ITO was not heat-treated before immersing in the colloid solution.

ature,followed by the immersion into the20-nm Au colloid solution.

3.2.Effects of heat treatment before immersing in20-nm Au

colloid solution

The aggregation of AuNPs is expected to be resulted by the resid-ual linker molecules on the surface after the?rst treatment

of Fig.2.Absorption spectra of AuNP-attached ITO substrates prepare using three linker molecules,i.e.(a)cysteamine,(b)MPTMS and(c)APTMS,via the one-step immersion into the20-nm Au colloid solution.The linker-modi?ed ITO was not heat-treated before immersing in the colloid solution.

ITO substrates in ethanol solutions.To remove the residual linker molecules,heat treatment is known to be effective.Actually,Wang and coworkers adopted80?C heat treatment for MPTMS[5,6]and 110?C treatment for APTMS[10].

In the present work,we tried a heat treatment at a lower temper-ature,40?C,and consequently,different results could be obtained in comparison with those obtained for untreated ITO samples,whose results are shown in Section3.1.Fig.3shows the SEM images of the AuNP-attached ITO surfaces prepared with heat treatment at40?C for1h and then immersing in the20-nm Au colloid solution.As in the local SEM images in Fig.3A and B,the aggregation was signif-icantly suppressed in the cases of cysteamine and MPTMS(Fig.3A and B)even at the low temperature treatment at40?C for1h.Fur-thermore,also in the case of APTMS,the connections of AuNPs have become less as shown in Fig.3C,which can be recognized in com-parison with Fig.1C.It is apparent that the treatment at40?C was effective to form well-dispersed AuNPs on the ITO surfaces.

The absorption spectra in Fig.4are again in good relation with the changes in the attachment of AuNPs of Fig.3.For the well-dispersed AuNP-attached ITO with the APTMS linker,the peak at the lower wavelength in Fig.4c has become distinct compared with the shoulder at the longer wavelength.The transformation from Fig.2c to Fig.4c should be a re?ection of the transformation from the connecting state to the dispersed state.The smaller absorptions of Fig.4a and b should be due to the decreased amount of attached AuNPs on the ITO surfaces as shown in Fig.3A and B.

The present results obtained for the attachment of AuNPs with the three linker molecules are merely one example obtained using the commercially available20-nm Au colloid solution as received. For each linker molecule,some different controls of the attachment would be possible by changing the experimental conditions,such as linking time and temperature,heat treatments,the concentration of colloid,etc.However,through the present systematic comparison, we can have general views as follows:

(1)APTMS is suitable,or effective,to attach the commercially avail-

able20-nm AuNPs on the ITO surface keeping a moderate dispersion of each NP.

(2)Compared with APTMS,the uses of MPTMS and cysteamine

tend to form the aggregation of AuNPs,so that some treat-ments would be necessary.However,heat treatment to prevent the aggregation would cause the decrease in the amount of the attached AuNPs.

As one conclusion until here,APTMS can be regarded as a good linker to attach AuNPs simply via the one-step immersion.

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Fig.3.Typical SEM images of AuNP-attached ITO surfaces prepare using three linker molecules,i.e.(A)cysteamine,(B)MPTMS and(C)APTMS,via the one-step immer-sion into the20-nm Au colloid solution.The linker-modi?ed ITO was heat-treated at40?C for1h before immersing in the colloid solution.

3.3.Attachment of AuNPs via the seed-mediated growth

treatment

Not only the simple one-step immersion,the nanostructuring from the AuNPs once attached on the surface would be very impor-tant for preparing some functional surfaces composed of AuNPs [6–9].Of course,the nanostructuring should depend on growth methods,as well as initial attached states of AuNPs.In the

present Fig.4.Absorption spectra of AuNP-attached ITO substrates prepared using three linker molecules,i.e.(a)cysteamine,(b)MPTMS and(c)APTMS,via the one-step immersion into the20-nm Au colloid solution.The linker-modi?ed ITO was heat-treated at40?C before immersing in the colloid solution.

work,the effects of the three linker molecules were explored by applying the seed-mediated growth method for surface modi?ca-tion[15].

In the seed-mediated growth method,the small Au nanoseed particles of ca.4nm can attach on the ITO surfaces via physisorp-tion.With the presence of the effective linker molecules,it is also expected that the Au nanoseeds reasonably attach on the ITO sur-faces.However,due to the limited resolution of the?eld-emission SEM used,the observation of the exact images after the seeding was not easy as mentioned in our previous paper[15].Thus,the SEM images were observed after the growth treatment followed by the seeding.

Fig.5shows the SEM images observed after the seed-mediated growth treatment.Interestingly,the results showed a remarkable contrast compared with the results obtained in the one-step https://www.wendangku.net/doc/0e2548285.html,ly,whereas the attachment and growth of AuNPs could be con?rmed on the MPTMS and cysteamine linker layers,the attached AuNPs were observed as very small as shown in Fig.5C. As the interpretation of the result of Fig.5C,it is inferred that the growth of Au nanoseeds was signi?cantly suppressed on the APTMS layer,whereas the growth was possible on the MPTMS or cysteamine layer.

The absorption spectra of Fig.6reasonably showed the differ-ences of the attachment of AuNPs in Fig.5.The intensity of the absorption re?ected the order of the attached amount of AuNPs, MPTMS>cysteamine>APTMS,as shown in Fig.5.The shapes of the absorption peaks became single-peak like shapes whose max-ima were at the shorter wavelength,which indicates that each AuNPs tend to well dispersed after the seed-mediated growth treatment,as recognized from the SEM images in Fig.5.This is because,in the seed-mediated growth treatment,not all the Au nanoseeds grew concurrently,but maturing to promote the growth of some bigger AuNPs occurred in the seeding process. The shoulder peak at a longer wavelength around750nm in Fig.6b would be a re?ection of some aggregation of AuNPs due to the denser attachment of growth of AuNPs on the MPTMS sur-faces.

Although the nanostructural growth should depend on the methods to grow the Au nanoseeds,at least in the seed-mediated growth method,it was found that MPTMS or cysteamine linker allows the growth of Au nanoseeds in the growth treatment,but that APTMS signi?cantly suppressed the growth.The results those showed the sharp contrast between the one-step immersion and the seed-mediated growth emphasize that the selection of linker molecules would be important depending on the purposes and the methods.

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Fig.5.Typical SEM images of AuNP-attached ITO surfaces prepared using three linker molecules,i.e.(A)cysteamine,(B)MPTMS and(C)APTMS,via the seed-mediated growth treatment,i.e.,the linker-modi?ed ITOs were?rst immersed into the seed solution for2h,and then into the growth solution for24h.

https://www.wendangku.net/doc/0e2548285.html,parison of the functions of three linker molecules

Judging from the results obtained after the one-step immer-sion into the20-nm Au colloid solution,the ability of the APTMS layer to grasp for individual AuNP would be expected to be much higher than those of the MPTMS and cysteamine layers.Thus,as for the result obtained for APTMS in Section3.3,it is expected that the holding of AuNPs on APTMS was too rigid to allow

the Fig.6.Absorption spectra of AuNP-attached ITO substrates prepared using three linker molecules,i.e.(a)cysteamine,(b)MPTMS and(c)APTMS,via the seed-mediated growth treatment.

structural growth.On the other hand,the attachment on MPTMS and cysteamine showed similar tendency both in the one-step and seed-mediated growth treatments,while the attached amount was higher on the MPTMS layer.

Therefore,if we would like to explain the characteristics of AuNPs’attachment simply based on the function group on the outer surface of the linker layer,the present results would not easily inter-preted,but rather show the confusion.It is because,APTMS and cysteamine,both of which have–NH2groups on the outer sur-face,showed the quite different results,and APTMS and MPTMS, both of which have the same main structure excepting the–NH2 and–SH groups,showed the different results,but MPTMS and cysteamine,which have different structures,showed the similar results.

This strange correlation would indicate that the attachment of AuNPs is not simply governed by the functional groups or chain length,but the complicated effects have been considered.The sizes of AuNPs are much larger than the area to be occupied by one functional group on the outer surface of the linker-modi?ed ITO, assuming the monolayer absorption(though it should depends on the linkers and preparation methods,it should be roughly less than 1nm×1nm).Thus,to estimate how many functional groups are involved to?x one AuNP is not easy.In addition,each AuNP is surrounded by capping reagents,whose amount or density should change depending on the particle size.Therefore,to propose exact models to explain the present results would be dif?cult.

However,as a rough image for the present result,it is inferred that APTMS having the–NH2group and the longer chain strongly grasp for AuNPs for both20-nm and4-nm AuNPs.Due to the strong interactions with the linker molecules,it is considered that the reductive growth is prohibited when Au nanoseeds were trapped on the APTMS layer.In contrast,the holding of AuNPs on MPTMS having–SH and cysteamine having–NH2but the shorter chain is inferred relatively weaker,so that the aggregation or the crystal growth are expected to occur.

More than the above rough images,some characterization as monolayer?lms of linker molecules would be necessary to con-clude the interactions,though the present work aimed the general comparison under the similar mild preparation conditions,which was successful to obtain the comparable results.

3.5.Electrochemical characteristics depending on linker

molecules

As one of the aims of the present work,the cysteamine was expected to be a good linker molecule because it has shorter chain.

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Fig.7.Cyclic voltammograms recorded using AuNP-attached ITO electrodes pre-pared using three linker molecules,i.e.(a)cysteamine,(b)MPTMS and(c)APTMS, via the one-step immersion into the20-nm Au colloid solution with the40?C heat treatment.The solution was phosphate buffer(pH7.4)contained1.0mM K4Fe(CN)6. Scan rate:0.05V/s.

However,the results obtained above have shown that the attached amounts of AuNPs were not necessarily large compared with the results with APTMS in the one-step immersion and with MPTMS in the seed-mediated growth treatment.So,the denser attach-ment of AuNPs on the cysteamine layer,further tuning of the preparation conditions would be necessary,and a simple com-parison might be dif?cult even if we observe the electrochemical responses.

For instance,Fig.7shows the differences in the electrochemi-cal responses on the AuNP-attached ITO substrates prepared with the heat treatment at40?C(whose surface images are those in Fig.3)as the working electrodes for a[Fe(CN)6]3?/[Fe(CN)6]4?couple.Although a distorted response was observed on the AuNP-attached MPTMS/ITO(Fig.7b),it showed a signi?cant recovery of Faradaic currents compared with the result with an MPTMS/ITO electrode(without attaching AuNPs),on which the electron transfer of the[Fe(CN)6]3?/[Fe(CN)6]4?couple was almost sup-pressed(data not shown).In contrast,the cysteamine/ITO and APTMS/ITO electrode showed almost reversible responses for the [Fe(CN)6]3?/[Fe(CN)6]4?couple even though without attaching AuNPs,which is probably due to the electrostatic effect of the –NH2group.Thus,simple evaluations of cysteamine as a linker for electron transfer reactions would not be easy because the electro-chemical responses should change depending on the redox couples as well as the changes in the attached density of AuNPs.

Concerning the cysteamine linker,some strange phenomena were observed as follows:

(1)In the electrochemical applications of AuNP-attached ITO

prepared on the cysteamine linker without heat treatment, the peak current in the reversed scan for the[Fe(CN)6]3?/ [Fe(CN)6]4?couple increased slightly(10–20%).So,the obtained voltammogram was no more reversible.

(2)When the AuNP-attached ITO prepared using the seed-

mediated growth and the cysteamine linker was left in air for1 week,the sizes of AuNPs on the surface have become bigger,i.e., the aggregation of AuNPs was occurred.This phenomenon was observed only for the seed-mediated grown AuNPs and on cys-teamine.It was recognized by FE-SEM and also from the change in the color of the ITO.

Both of the above results should re?ect the instability of cysteamine layers.Thus,further investigations are now in progress concerning the stability and applicability of cysteamine as the linker molecule in electroanalysis.4.Conclusions

As the results of the present work,which was initially aimed to evaluate the characteristics of cysteamine as a linker molecule for AuNPs and ITO,we could notice the AuNPs’nanostructures formed on the ITO surfaces signi?cantly depend on the linker molecules even if we use the similar treatments in mild conditions.In com-parison with the results obtained with the MPTMS and cysteamine linkers,it was characteristic that APTMS was found to grasp for AuNPs without permitting the gathering in the one-step attach-ment of20-nm AuNPs or the nanostructural growth of AuNPs in the seed-mediated growth method.

If we would like to attribute the above results of APTMS def-initely to the functional group,–NH2,the results of cysteamine having the same–NH2terminal,are dif?cult to explain.Although the exact reasons of the differences between APTMS and cys-teamine are not clear,it would be true that,when we select linker molecules,the attachment cannot be simply expected by the func-tional groups to link AuNPs.

The differences in the results of APTMS and MPTMS would imply that a strategically selection is important depending on the pur-poses.That is,APTMS linker would be better to attach de?ned sized AuNPs in a simple treatment,and MPTMS may be better to try the nanostructural growth.However,the latter would depend on the methods of the nanostructuring or the capping reagents of Au nanoseed particles.Now,we are investigating the nanostruc-tural growth from the commercially available tannic acid capped Au nanoseeds.

Finally,although the conclusions obtained from the present results were not straightforward and the applicability of cysteamine as the linker could not be clearly shown,we believe systematic com-parisons as carried out in the present work should be important to understand the fundamental aspects of nanostructuring involving metal NPs.

Acknowledgements

This work was supported in part by a Grant-in-Aid for Scienti?c Research(C),No.20550074from the Ministry of Education,Cul-ture,Sports,Science and Technology(MEXT),Japan.K.N.is thankful to the Japan Society for the Promotion of Science(JSPS)for the fellowship.

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[21]J.W.Slot,H.J.Geuze,Eur.J.Cell Biol.38(1985)87.

on the contrary的解析

On the contrary Onthecontrary, I have not yet begun. 正好相反，我还没有开始。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, the instructions have been damaged. 反之，则说明已经损坏。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I understand all too well. 恰恰相反，我很清楚 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I think this is good. ⑴我反而觉得这是好事。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I have tons of things to do 正相反，我有一大堆事要做 Provided by jukuu Is likely onthecontrary I in works for you 反倒像是我在为你们工作 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, or to buy the first good. 反之还是先买的好。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, it is typically american. 相反，这正是典型的美国风格。 222.35.143.196 Onthecontrary, very exciting.

恰恰相反，非常刺激。 https://www.wendangku.net/doc/0e2548285.html, But onthecontrary, lazy. 却恰恰相反，懒洋洋的。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I hate it! 恰恰相反，我不喜欢！ https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, the club gathers every month. 相反，俱乐部每个月都聚会。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I'm going to work harder. 我反而将更努力工作。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, his demeanor is easy and nonchalant. 相反，他的举止轻松而无动于衷。 https://www.wendangku.net/doc/0e2548285.html, Too much nutrition onthecontrary can not be absorbed through skin. 太过营养了反而皮肤吸收不了. https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, I would wish for it no other way. 正相反，我正希望这样 Provided by jukuu Onthecontrary most likely pathological. 反之很有可能是病理性的。 https://www.wendangku.net/doc/0e2548285.html, Onthecontrary, it will appear clumsy. 反之，就会显得粗笨。 https://www.wendangku.net/doc/0e2548285.html,

英语造句

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●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the

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介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词第二章节：英语句子成分主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。定语：只对名词起限定修饰的成分，一般由形容

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M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.

base on的例句

意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机突袭空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.

英语造句大全

英语造句大全English sentence 在句子中，更好的记忆单词！ 1、(1)、able adj. 能句子：We are able to live under the sea in the future. (2)、ability n. 能力句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地句子：His calculation is accurately. 3、(1)、act v. 扮演句子：He act the interesting character. （2）、actor n. 演员句子：He was a famous actor. (3)、actress n. 女演员句子：She was a famous actress. (4)、active adj. 积极的句子：He is an active boy. 4、add v. 加句子：He adds a little sugar in the milk. 5、advantage n. 优势句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n.

(完整版)主谓造句

主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。

初中英语造句

【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early

The Kite Runner-美句摘抄及造句

《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high

翻译加造句

一、翻译 1. The idea of consciously seeking out a special title was new to me., but not without appeal. 让我自己挑选自己最喜欢的书籍这个有意思的想法真的对我具有吸引力。 2.I was plunged into the aching tragedy of the Holocaust, the extraordinary clash of good, represented by the one decent man, and evil. 我陷入到大屠杀悲剧的痛苦之中，一个体面的人所代表的善与恶的猛烈冲击之中。 3.I was astonished by the the great power a novel could contain. I lacked the vocabulary to translate my feelings into words. 我被这部小说所包含的巨大能量感到震惊。我无法用语言来表达我的感情（心情）。 4，make sth. long to short长话短说 5.I learned that summer that reading was not the innocent(简单的) pastime(消遣) I have assumed it to be., not a breezy, instantly forgettable escape in the hammock(吊床)，( though I’ ve enjoyed many of those too ). I discovered that a book, if it arrives at the right moment, in the proper season, will change the course of all that follows. 那年夏天，我懂得了读书不是我认为的简单的娱乐消遣，也不只是躺在吊床上，一阵风吹过就忘记的消遣。我发现如果在适宜的时间、合适的季节读一本书的话，他将能改变一个人以后的人生道路。二、词组造句 1. on purpose 特意，故意 This is especially true here, and it was ~. (这一点在这里尤其准确，并且他是故意的) 2.think up 虚构，编造，想出 She has thought up a good idea. 她想出了一个好的主意。 His story was thought up. 他的故事是编出来的。 3. in the meantime 与此同时助记：in advance 事前in the meantime 与此同时in place 适当地... In the meantime, what can you do? 在这期间您能做什么呢？ In the meantime, we may not know how it works, but we know that it works. 在此期间，我们不知道它是如何工作的，但我们知道，它的确在发挥作用。 4.as though 好像，仿佛 It sounds as though you enjoyed Great wall. 这听起来好像你喜欢长城。 5. plunge into 使陷入 He plunged the room into darkness by switching off the light. 他把灯一关，房

改写句子练习2标准答案

The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design.

用以下短语造句

M1 U1 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： add up (to) be concerned about go through set down a series of on purpose in order to according to get along with fall in love (with) join in have got to hide away face to face 1 We’ve chatted online for some time but we have never met ___________. 2 It is nearly 11 o’clock yet he is not back. His mother ____________ him. 3 The Lius ___________ hard times before liberation. 4 ____________ get a good mark I worked very hard before the exam. 5 I think the window was broken ___________ by someone. 6 You should ___________ the language points on the blackboard. They are useful. 7 They met at Tom’s party and later on ____________ with each other. 8 You can find ____________ English reading materials in the school library. 9 I am easy to be with and _____________my classmates pretty well. 10 They __________ in a small village so that they might not be found. 11 Which of the following statements is not right ____________ the above passage? 12 It’s getting dark. I ___________ be off now. 13 More than 1,000 workers ___________ the general strike last week. 14 All her earnings _____________ about 3,000 yuan per month. 二.用以下短语造句： 1.go through 2. no longer/ not… any longer 3. on purpose 4. calm… down 5. happen to 6. set down 7. wonder if 三. 翻译： 1.曾经有段时间，我对学习丧失了兴趣。（there was a time when…） 2. 这是我第一次和她交流。（It is/was the first time that …注意时态） 3.他昨天公园里遇到的是他的一个老朋友。（强调句） 4. 他是在知道真相之后才意识到错怪女儿了。（强调句） M 1 U 2 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： play a …role (in) because of come up such as even if play a …part (in) 1 Dujiangyan(都江堰) is still ___________in irrigation(灌溉) today. 2 That question ___________ at yesterday’s meeting. 3 Karl Marx could speak a few foreign languages, _________Russian and English. 4 You must ask for leave first __________ you have something very important. 5 The media _________ major ________ in influencing people’s opinion s. 6 _________ years of hard work she looked like a woman in her fifties. 二.用以下短语造句： 1.make (good/full) use of 2. play a(n) important role in 3. even if 4. believe it or not 5. such as 6. because of

英语造句

English sentence 1、(1)、able adj. 能句子：We are able to live under the sea in the future. (2)、ability n. 能力句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地句子：His calculation is accurately. 3、(1)、act v. 扮演句子：He act the interesting character.（2）、actor n. 演员句子：He was a famous actor. (3)、actress n. 女演员句子：She was a famous actress. (4)、active adj. 积极的句子：He is an active boy. 4、add v. 加句子：He adds a little sugar in the milk. 5、advantage n. 优势句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 句子：He is in America. 10、appear 出现v. He appears in this place. 11. artist 艺术家n. He is an artist. 12. attract 吸引 He attracts the dog. 13. Australia 澳大利亚 He is in Australia. 14.base 基地 She is in the base now. 15.basket 篮子 His basket is nice. 16.beautiful 美丽的 She is very beautiful. 17.begin 开始 He begins writing. 18.black 黑色的 He is black. 19.bright 明亮的 His eyes are bright. 20.good 好的 He is good at basketball. 21.British 英国人 He is British. 22.building 建造物 The building is highest in this city 23.busy 忙的 He is busy now. 24.calculate 计算 He calculates this test well. 25.Canada 加拿大 He borns in Canada. 26.care 照顾 He cared she yesterday. 27.certain 无疑的 They are certain to succeed. 28.change 改变 He changes the system. 29.chemical 化学药品