Solvent: A Key in Digestive Ripening for Monodisperse Au Nanoparticles A Key in Digestive Ripening...
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NANO EXPRESS Open Access
Solvent: A Key in Digestive Ripening forMonodisperse Au NanoparticlesPeng Wang1, Xuan Qi1, Xuemin Zhang1, Tieqiang Wang1, Yunong Li1, Kai Zhang1, Shuang Zhao1*,Jun Zhou1,2* and Yu Fu1*
This work has mainly investigated the influence of the solvent on the nanoparticles distribution in digestiveripening. The experiments suggested that the solvents played a key role in digestive ripening of Au nanoparticles(Au NPs). For the benzol solvents, the resulting size distribution of Au NPs was inversely related to the solventpolarity. It may be interpreted by the low Gibbs free energy of nanoparticles in the high polarity medium, whichwas supposedly in favor of reducing the nanoparticles distribution. Through digestive ripening in the highly polarbenzol solvent of p-chlorotoluene, monodisperse Au NPs with relative standard deviation (RSD) of 4.8% wereachieved. This indicated that digestive ripening was an effective and practical way to prepare high-qualitynanoparticles, which holds great promise for the nanoscience and nanotechnology.
Keywords: Solvent polarity, Digestive ripening, Au nanoparticles, Monodispersity
BackgroundAu nanoparticles (Au NPs) and their self-assemblieshave drawn intense attentions due to their uniqueproperties and potential applications [1, 2], such as cata-lysts , electronic and optoelectronic nanodevices ,biosensors , and biomedicine . For fully implement-ing the functions of Au NPs, the uniform size distributionis of key significance. The monodisperse nanoparticles(relative standard deviation less than 5.0%) show uniqueproperties and higher performances compared with thecorresponding polydisperse ones [7, 8]. Unfortunately, it ishard to prepare monodisperse Au NPs with simple syn-thesis process and common starting materials .Therefore, a great deal of effort has been made to obtainmonodisperse Au NPs . One main strategy is direct al-teration of the synthesis method, including selection ofspecial metal sources and reduction agents, introductionof nanoparticle seeds, addition of surfactants, and so on.However, although the monodisperse Au NPs have beenachieved by direct synthesis , it suffers from high-costof starting materials, tedious separation and unsatisfied
repeatability. An alternative approach to preparing mono-disperse Au NPs is post-treatment of the prepared ones,which could detour the issue.Digestive ripening, discovered by Lin , is an effect-
ive post-treatment method. It is carried on by refluxinga nanoparticle suspension with an excess amount of cap-ping agents (called as digestive ripening agents (DRA)),which could cause the shrink of large particles and thegrowth of small particles to achieve an equilibrium sizeat a stable state [11, 12]. A distinct feature of digestiveripening is that it can obtain high reproducibility andyield with fine control . The effect factors on digest-ive ripening, including capping agent, temperature,refluxing time, field effect, and the length of digestiveripening agent, have been explored [11, 1620]. Solventis an important participant in digestive ripening. More-over, the study has shown the solvent plays a vital role inthe synthesis process, which implied the effect of thesolvent on digestive ripening would be remarkable.However, the related work has been rarely reported. Inaddition, thus far the monodisperse Au nanoparticlesobtained by digestive ripening mainly came from thesystem reported by Lin . In that system, the originalAu nanoparticles were synthesized by a surfactant-assisted method. Its process was tedious and the yield
* Correspondence: email@example.com; firstname.lastname@example.org;email@example.comEqual contributors1College of Sciences, Northeastern University, Shenyang 110004, ChinaFull list of author information is available at the end of the article
The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.
Wang et al. Nanoscale Research Letters (2017) 12:25 DOI 10.1186/s11671-016-1797-7
was low, that substantially weakened the advantage ofdigestive ripening.Based on the above consideration, digestive ripening
of Au NPsin different solvents was investigated in thiswork. The experiments showed that there was no obvi-ously development in size distribution of Au NPs in thesolvents of linear hydrocarbon. However, there was adramatic change when benzol solvents were used. More-over, the distribution was closely related with the solventpolarity. The higher the polarity of the used solvent is,the lower the RSD of the resulting nanoparticles is.When a highly polar benzol of p-chlorotoluene was usedas the reflux solvent, RSD of the Au NPs after digestiveripening could reach as low as 4.8%, with which thesuperlattices could be assembled. Additionally, it isworth to emphasize that the original preparation of theAu NPs in our study was fast, low-costed, andsurfactant-free. Combined with the simple synthesismethod, the work offered a promising approach to facilefabrication of the monodisperse Au nanoparticles.
MethodsSynthesis of Initial Au NPsThe DDT-capped gold NPs was synthesized according tothe literature . Firstly, 50 mM AuCl4
/H+ solutionwas prepared by dissolving HAuCl44H2O with the samemolar amount of HCl in aqueous. 50 mM BH4
/OH so-lution was made by dissolving NaBH4 with the samemolar amount of NaOH in aqueous. Secondly, 475 L ofAuCl4
/H+ solution was diluted with 47 mL DI water.Then, BH4
/OH (2.125 mL of 50 mM) was added to thesolution under magnetically stirring, followed by heatingfor 34 min at boiling temperature of water. After it wascooled to room temperature, acetone (32 mL), hexane(38 mL), and DDT (0.5 L) was introduced into the so-lution and mixed vigorously by hands for about 1 min totransfer Au NPs to the organic phase. Finally, the mixedsolution was separated by a separating funnel, and theinitial Au NPs were obtained.
Digestive Ripening of Au NPsIn a typical digestive ripening, a batch of as-prepared AuNPs solution was evaporated for about 35 C by rotaryevaporation to get rid of the solvent. And then, the driedAu NPs were re-dispersed in 10 mL different solvents(n-hexane, toluene, m-xylene, p-xylene, chlorobenzene,p-chlorotoluene and n-octane) with the addition of2.5 L DDT, respectively. Those colloids were refluxedat boiling point of different solvents and then slowlycooled down. The whole process of digestive ripeningwas completed.
Results and DiscussionIn this study, we chose a two-step method to synthesizeDDT-coated Au NPs. The first step was preparing nakedAu NPs in an aqueous solution by reduction of HAuCl4by NaBH4. The second was coating the NPs with DDTby extracting the naked NPs into a DDT solution in hex-ane. Thanks to the low-costed starting materials and fa-cile process, this method was an effective way to prepareat a large scale. Unfortunately, the polydispersity of theresulting nanoparticles was still unsatisfied. As shown inFig. 1, the average diameter of the Au NPs was 4.47 nmand the RSD was 15%.Because NaBH4was used as the reduction agent, the
synthesis process of the Au nanoparticles was fast andalmost uncontrollable; it would be difficult to further im-prove the distribution by optimizing synthesis conditions.Therefore, post-treatment of the resulting nanoparticleswas an alternative strategy to improve the size distribu-tion. In this work, we employed the digestive ripening asthe post-treatment and mainly explored the influence ofthe solvents. Digestive ripening was carried on by reflux ofthe prepared Au NPs in different solvents (n-hexane, tolu-ene, p-xylene, m-xylene, chlorobenzene, and n-octane) attheir respective boiling temperatures. The TEM imagesand corresponding distribution analysis of the Au NPsafter 4 h digestive ripening were shown in Fig. 2, and therelated data were summarized in Table 1.
Fig. 1 a TEM image of the initial Au nanoparticles. b The corresponding particle size histogram
Wang et al. Nanoscale Research Letters (2017) 12:25 Page 2 of 7
In digestive ripening of the Au NPs, there were twopossible factors that could play the vital role. The firstpossible one was the digestive ripening temperature, i.e.,the reflux temperature of the solvent. However, asshown in Table 1, there was not a direct relationship be-tween the reflux temperature and the size distribution.For example, the reflux temperatures of p-xylene, m-xy-lene, and chlorobenzene were almost the same, but thesize distribution of the Au NPs after digestive ripeningin them was completely different, ranging from 9.5 to17%. For the other solvents, including n-hexane, toluene,
and n-octane, the distribution also varied with the refluxtemperatures at random suggesting that the digestiveripening temperature was not directly related with thesize distribution.The second possible factor could be the physical prop-
erty of the solvent. The solvent we used in digestive ripen-ing can be classified into two categories: linear solventsand benzol solvents. As shown in Table 1, the linear sol-vents (n-hexane, n-octane) could not remarkably improvethe nanoparticles distribution. This was likely because theflexible chain of linear solvents could intertwine with the
Fig. 2 TEM images of the Au NPs after 4 h digestive ripening in di