Synthesis of low-cost 3D-porous ZnO/Ag SERS-active substrate with

ultrasensitive and repeatable detectability

Maosen Yanga, Jing Yua, b†, Fengcai Leic, d†, Hang Zhoua, Yisheng Weia, Baoyuan Mana, Chao

Zhanga, b, Chonghui Lia, Junfeng Rena, b, and Xiaobo Yuana†

aSchool of Physics and Electronics, Shandong Normal University, Jinan, 250014, P.R. China.

bInstitute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, P.R. China.

cCollege of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan,

250014, P.R. China.dInstitute of Biomedical Sciences, Shandong Normal University, Jinan 250014, P.R. China.

†Corresponding e-mail: yujing1608@126.com, leifc@sdnu.edu.cn, yxb@sdnu.edu.cn

Discussions

In Fig. S5, it’s observed that as the size of Ag NPs increases, the intensity of R6G at 613 cm-1 rises

first and then declines. This phenomenon mainly drives from the transition between near- and far-

field effect for LSPR: As we know, both of them contribute well to SERS enhancement, but, the

near-field effect, by contrast, is more outstanding. When the NPs is not big, near-field effect stands

in a dominant position, and conversely the far-field effect has the call. However, if the size of NPs

is too small, most incident lights will be converted into heat, which is helpless for the SERS

enhancement, thus the initial Raman intensity is relatively low. As NPs become bigger, the

photothermal effect recedes gradually, and subsequently the lights are limited around the NPs and

present as the local enhanced electric field, i.e., the well-known ‘hot spots’. So, the intensity

begins to rise. If the particle’s size continues to increase, far-field effect appears gradually. But, the

improved far-field effect don’t restrain the near-field effect so much at the very start, therefore

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benefitting from the synergy of these two effects, the intensity goes on rising. Finally, when the

size becomes bigger again, the suppression by far-field effect gets much stronger, thus the

intensity declines then.

Surface coverage (SC) of Ag NPs is another key factor for SERS enhancement, and in this paper,

as the SC increases the intensity will increase first and then tend to constant if we keep the

particle’s size unchanged. For the low SC, the Raman intensity is very weak, because there are

insufficient Ag NPs to provide the ‘hot spots’. Therefore, in the initial stage, increasing the density

of Ag NPs will bring better SERS performance. But, for the excessive-high SC, too many Ag NPs

on the surfaces will hinder lights to enter into the deeper position of the substrate, which finally

leads to that only superficial ZnO/Ag heterostructure is utilized. Although more Ag NPs will

provide more ‘hot spots’, many of them are inoperative, thus the Raman intensity will not rise

again, and at last tends to constant.

In this paper, the optimal size and SC are ca. 75 nm and 60 %, respectively. All the results

discussed in the main body, unless otherwise indicated, are all at this condition.

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Figures and Tables

Fig. S1. SEM image of the 3D-porous ZnO framework without Ag NPs

Fig. S2. the heterostructure of Ag and ZnO.

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Fig. S3. the size distribution of Ag NPs.

Fig. S4. time-dependence Raman mappings for R6G (10-8 M, wavenumber of 613 cm-1) on the 3D-porous ZnO/Ag substrate under natural (I: a-c) and dark environments (II: d-f), respectively.

Fig. S5. the Raman intensity of 3D-porous ZnO/Ag bases with various Ag NPs: the blue line

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represents Ag NPs with different concentrations (size is kept at ca. 72 nm) and the red line relates to Ag NPs with various sizes (surface coverage is kept at ca. 60 %); the target molecule is R6G of 10-8 M, and the intensity refers to wavenumber of 613 cm-1.

Fig. S6. Raman intensity of (a) CV at 913 cm-1, (b) MG at 1617 cm-1, (c) Sudan I at 727 cm-1for different concentrations

Table SI. vibrational modes for characteristic peaks of CV, MG and Sudan I, respectively (10-7 M)Raman shifts (cm-1) Intensity (a.u.) Vibrational mode

913 (CV)1176 (MG)

12125872

C-H out-of-plane bending modeC-H in-plane bending mode

1226 (Sudan I) 2170 C-H in-plane bending mode

1175 (CV)1367 (MG)

1385 (Sudan I)

1241424451870

C-H in-plane bending modeN-phenyl stretching mode

C-H in-plane bending mode

1620 (CV)1617 (MG)

1495 (Sudan I)

132143123990

aromatic C-C stretching modering C–C stretching vibrationring C-C stretching vibration

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