Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal

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Functional Nanostructured Materials (including low-D carbon)

Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal Nanoparticles for SERS-based Detection of Trace Heavy Metal Ions. Haoming Bao, Hongwen Zhang, Le Zhou, Hao Fu, Guangqiang Liu, Yue Li, and Weiping Cai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05878 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal Nanoparticles for SERS-based Detection of Trace Heavy Metal Ions Haoming Baoa, b, Hongwen Zhanga*, Le Zhoua, b, Hao Fua, b, Guangqiang Liua, Yue Lia and Weiping Cai a, b* a

Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute

of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China b

University of Science and Technology of China, Hefei 230026, PR China

KEYWORDS: Thiourea-induced isotropic shell growth; Ultrathin sulfide wrapping; Plasmonic metal nanoparticles; SERS-based detection; Trace heavy metal ions. ABSTRACT: A facile and general strategy is presented for homogenous and ultrathin metal sulfide-wrapping on plasmonic metal (PM) nanoparticles (NPs) based on a thiourea-induced isotropic shell growth. This strategy is typically implemented just via adding the thiourea into pre-formed PM colloidal solutions containing target metal ions. The validity of this strategy is demonstrated by taking the wrapped NPs with Au core and CuS shell or Au@CuS NPs as an example. They are successfully fabricated via adding the thiourea and Cu2+ solutions into pre-

*

To whom all correspondence should be addressed E-mail: [email protected], [email protected]; Tel: +86-551-65592747; Fax: +86-551-65591434

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formed Au NP colloidal solution. The CuS shell layer is highly homogenous ( 10 nm in thickness), which is not beneficial to making use of the short range 3 ACS Paragon Plus Environment


(generally < 5 nm) SPR effect of the PM cores. As for the cation ion exchange procedure, it strictly limits the scope of the MS shell due to the selective and directional occurrence for the ion exchange reactions although the relatively uniform and thin shells could be obtained. So far, to our best knowledge, only CdS, PbS and ZnS wrapping layers have been reported. The controlled fabrications for the ultrathin (several nanometers) and homogeneous MS layer-wrapped PM NPs are still expected and in challenges. Herein, a facile and general route is presented to wrap the PM NPs with a uniform and ultrathin MS layer based on the thiourea-induced isotropic shell growth just by adding the thiourea solution into the pre-formed PM colloidal solution containing the target metal ions. The validity of this route has been demonstrated by the fabrication of Au@CuS NPs. The Au@CuS NPs are obtained via adding thiourea solution into the pre-formed Au NPs’ colloidal solution containing enough Cu2+ ions. The CuS shells are highly homogenous in thickness, regardless of the Au NPs’ shapes or curvatures, due to the isotropic growth of CuS on the Au NPs. The shell thickness can be tuned from tens down to 0.5 nm just by controlling the added amount of the shell precursors (or the thiourea and Cu2+). Moreover, this route is of good universality. Many other sulfide-wrapped PM NPs can also be fabricated, such as Ag@CuS, Au@PtS2, Au@HgS, and Ag@Ag2S NPs as well as Ag@CuS NRs. Importantly, such ultrathin sulfide layer-wrapped PM NPs can be used as SERS substrates for detection of trace heavy metal ions in solutions with strong environment anti-interference. The details are reported in this article. 2. STRATEGY Obviously, if the shell layer grows isotropically, the homogenous and ultrathin wrapping layer is expected to be achieved. It well known that thiourea molecules contain the amine 4 ACS Paragon Plus Environment

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groups, which can interact strongly with PM atoms, and can be adsorbed on the surface of PM NPs, leading to the formation of the thiourea molecules’ monolayer with the exposed sulfur atoms.29, 30 The exposed sulfur atoms can capture and react with the metal ions to form MSs. On this basis, here we present a thiourea-induced isotropic growth strategy, as illustrated in Scheme 1.

Scheme 1 The schematic illustration for the thiourea-induced isotropic growth strategy. (a): A PM NP in the aqueous solution with target metal ions. (b): Thiourea molecules’ monolayer with the exposed sulfur atoms are formed on the PM NP and the target metal ions are captured by the exposed sulfur atoms. (c): Formation of a PM@MS NP after the alternative deposition of the metal ions and sulfur ions on the PM NP.

When the enough thiourea molecules are added into the PM NPs’ colloidal solution with enough target metal ions, the partial dissolution occurs to form S2- ions in the solution due to the hydrolysis, and the surface of the PM NPs would be covered with monolayer thiourea molecules via adsorption, as shown in Scheme 1a, b. The exposed sulphur atoms on the PM NPs can capture the metal ions, which would, in turn, capture the S ions. Considering the difficulty of the homogeneous nucleation in the solution,31 the metal ions and sulfur ions would be preferentially and alternatively deposited on the PM NPs, and the synchronous and isotropic growth of MS shell thus occurs, resulting in the formation of the homogeneous MS wrapping layer, as shown in Scheme 1c. 5 ACS Paragon Plus Environment


Due to the relatively small size, the thiourea molecules can be homogeneously (in nanoscale) absorbed on whole PM NPs’ surface, regardless of the PM NPs’ shapes or curvatures. This could bring an isotropic and homogeneous growth of the wrapping shells on the differently shaped PM NPs. Also, by adjusting the amount of shell precursors (or the thiourea and target metal ions), the wrapping layer could be tuned and controlled within an ultrathin thickness. Furthermore, since many metal cations (such as Pt4+, Hg2+, Ag+ and Cd2+, etc.) can be strongly bonded with sulfur, it is expected that the above strategy could be of universality and used for fabrication of the various MS wrapping layers, which have been confirmed in this paper. Firstly, we use the CuS layer-wrapped Au NPs to demonstrate the validity of this strategy. 3. EXPERIMENTAL METHODS 3.1. Materials. The auric chloride acid (HAuCl4), sodium citrate, tris (hydroxymethyl) aminomethane (TA), thiourea, copper dichloride (CuCl2), ethanol, chloroplatinic acid (H2PtCl6), and mercury chloride (HgCl2) were bought from Alfa Aesar Corporation, and used without further purification. Deionized water was produced in a Milli-Q system and about 18.2 MΩ cm in resistivity. 3.2. Fabrication of Au@CuS NPs. The Au@CuS NPs were prepared by adding thiourea solution into the pre-prepared Au NPs’ colloidal solution containing enough Cu2+ ions according to the strategy shown in Scheme 1. 3.2.1. Preparation of Au NPs. The Au NPs was fabricated via a seed-mediated method according to a previous work.32 In detail, firstly, the Au seeds were prepared by the sodium citrate reduction method. Typically, 48 mL of boiling water was added with 6 ACS Paragon Plus Environment

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HAuCl4 solution (0.5 mL, 25 mM) and stirred for 10 min before rapidly injecting sodium citrate (1.5 mL, 1 %wt). After boiling for 10 minutes and natural cooling, the seeds’ solution was achieved with 17 nm in size and 9.1×1011 mL-1 in number concentration. For preparation of Au NPs, 47.5 mL of boiling water was then added with 2 mL of aqueous TA solution (0.1 M) and gently stirred for 5 min, before successively adding 0.1 mL of the seed solution and 0.3 mL of HAuCl4 (25 mM) solution. The mixed solution was kept boiling for 30 min before cooling to room temperature. The Au colloidal solution was thus obtained for further use. 3.2.2. CuS layer wrapping. CuS wrapping on Au NPs were then carried out. Typically, 500 L of CuCl2 solution (1% wt) was firstly added into the as-prepared 50 mL of Au colloidal solution with gentle stirring. The pH value in the solution was about 7. Then 100 L of thiourea aqueous solution (1% wt) was rapidly added into the mixed solution before standing for reaction under the ambient condition (the volume ratio of thiourea to CuCl2 solutions is 1:5). After reaction for 2 h, the products were collected and centrifugally cleaned before re-dispersion in 20 mL of pure water. 3.3. Characterization. The extinction spectral measurements were conducted on a Shimadzu UV-2600 spectrometer. The morphological characterization was carried out for the products on a field emission scanning electron microscope (FESEM, FEI Sirion 200). Microstructural examination was performed on a transmission electron microscope (TEM, FEI Tecnai G2 F20, USA) equipped with an energy dispersion spectroscope (EDS, Oxford IE250X-Max50). The Raman spectra were recorded by a confocal Raman



spectrometer (RENISHAW Invia Raman Microscope) with 1.7 mW/m2 in power, 633 nm in excitation wavelength and 5 s in integral time. 4. RESULTS AND DISCUSSION Au colloidal solution with rosy color was obtained via the seed-mediated method, as shown in inset I in Figure 1. The corresponding optical absorbance spectrum shows a peak at 550 nm or the SPR of Au NPs (Fig. 1). FESEM observation reveals that Au NPs are of equiaxial shape and nearly mono-dispersed with about 45 nm in size (Fig. S1a).

Fig. 1 Normalized optical absorbance spectra of colloidal solutions, and their corresponding real photos. Curves or photos (I) and (II) correspond to the as-prepared Au colloidal solution before and after adding Cu2+ ions and thiourea into it for 2 h, respectively.

4.1. Uniform and ultrathin CuS layer wrapped Au NPs. After addition of thiourea in the Au colloidal solution with Cu2+, the SPR of the Au NP colloidal solution was 8 ACS Paragon Plus Environment

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significantly red-shifted from 550 nm to 605 nm with the reaction time up to 2 h, accompanying with the color change from the rosy to the blue, as shown in Fig. 1 and Fig. S1b. The longer reaction induced the nearly unchanged SPR and solution color, indicating the complete reaction after 2 h (Fig. S1b). The significant red-shift of the SPR should be attributed to the change of dielectric environment around Au NPs,33, 34 indicating that the Au NPs may be wrapped with dielectric matter. Fig. 2a shows a FESEM image of the products after reaction for 2 h. The products are composed of the particles with a mean size of about 50nm. These particles are nearly monodispersed with quasi-spherical shape, similar to the bare Au NPs before reaction (Fig. 2a and Fig. S1a). The TEM observation reveals that the NPs are constructed in well-defined core-shell structure, and the wrapping layer is very uniform and about 4.4 ± 0.4 nm in thickness, as illustrated in Fig. 2b, c. The locally magnified image reveals that even on the NPs’ surface with varying curvatures, no discernible thickness changes can be observed. Fig. 2d gives the EDS results of the wrapped NPs and shows the existence of elements Au, Cu, S, and Ni, in which the element Ni should come from the nickel grid for supporting the sample in TEM observation. The atomic ratio of Cu: S was about 1.1: 1. This is close to that in CuS. Further, Fig. 2e presents the mapping of the elements Au, Cu and S in an isolated wrapped NP. It can be seen that the distribution area of Au is slightly smaller than that of Cu and S. This indicates that Au element is mainly distributed in the core part, while Cu and S are mainly distributed in the shell layer. These suggest that the shell is composed of CuS. The high resolution (HR) TEM examination was carried out for a wrapped Au NP, as shown in Fig. 2f. The lattice fringes with 0.23 nm in interplanar spacing in the core part match well with



Au {111} crystal planes, and those with 0.32 nm in the shell layer correspond to the crystal planes {100} of hexagonal CuS phase (PDF No. 06–0464).

Fig. 2 The characterizations of Au@CuS NPs. (a): A FESEM image. (b): A TEM image. The inset: the local magnification of a NP. (c): Distribution of the shell’s thickness [data from (b)]. (d): The EDS spectrum corresponding to (b). (e): The mapping of the elements Au, S, and Cu in an isolated wrapped NP. (f): HRTEM image of a local area near the surface layer of an isolated wrapped NP.

4.2. Tunable shell thickness. Further, the controllability in the shell’s thickness has been investigated for such CuS-wrapped Au NPs. It has been found that the thickness could be controlled just by adjusting the amounts of the added CuCl2 and thiourea solutions. Here, the volume ratio of the CuCl2 solution (1% wt) to the thiourea solution (1% wt) was fixed at 5:1. It is found that the shell thickness increases with the volume of the added thiourea solution, showing a good linear relationship between them, as illustrated in Fig. 3a. Correspondingly, the SPR of the wrapped NP colloidal solution redshifts and broadens with the shell thickness increasing, as shown in Fig. 3b, c. This is 10 ACS Paragon Plus Environment

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attributed to the increasing dielectric capacity of the wrapping layer and the growing coupling between the excitonic state of CuS shell and the plasmonic mode of the Au core, respectively.35 Also, the TEM observations reveal that all the wrapped NPs are of the homogenous shells, as typically shown in Fig. 3d. Fig. 3e shows the statistical thickness distributions for the CuS-wrapped Au NPs with the different shell thicknesses. The relative standard deviation (RSD) is

Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal

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