In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

Jing Yu; Yisheng Wei; Huijie Wang; Chao Zhang; Yunjia Wei; Minghui Wang; Baoyuan Man; Fengcai Lei

doi:10.1364/OE.27.009879

1. Introduction

How to realize a fast and efficient detection of trace hazardous chemicals in water, related closely to human health and environmental security, is always a thorny problem for people [1]. Among various detection techniques, surface-enhanced Raman scattering (SERS) spectroscopy has drew lots of attentions from researchers in chemical and biochemical fields, due to its unique fingerprint-identification characteristic and applications to understand surface chemistry at the nanoscale [2–4]. In the past decade, although many works have been down to develop this technology e.g. single molecule detection, time-resolved detection, etc [5–8], improving the sensitivity of SERS substrates to obtain a lowest limit of detection (LOD) of target molecules is still the most important point researchers care about [9,10]. However, in terms of sensitivity alone, many of the known LODs have been sufficiently low recently. For example, Yang et al. realized a ultrasensitive detection for Rhodamine 6G (R6G) of 10⁻¹⁸ mol/L by constructing a superhydrophobic SERS substrate [11]; Lei’s group also obtained a LOD of 10⁻¹⁸ mol/L for R6G on the Au@Al₂O₃@Au arrays with a enhanced factor (EF) as high as 10¹² [12]; by utilizing periodic metal hexangular array with subnanometer gaps, Xiao et al. detected the Hg²⁺ of 8.3 × 10⁻⁹ mol/L successfully [13]. However, besides the excellent detectability of the recent SERS technique, the methods of preparing high-performance SERS substrates reported in literature are usually too cumbersome. For instance, in Xiao's report, to build a periodic array, a lot of work needs to be done, such as nanosphere self-assembly, metal film deposition, nanosphere removal, etc., and further consider the preparation of the entire structure, the transfer of graphene is also difficult. Thus, the sensitivity is no longer a major obstacle for SERS substrate. How to simplify the preparation process and obtain an economic and practical SERS substrate simultaneity becomes an urgent issue for researchers, which is also the main motivation of this paper.

To design a cost-effective SERS substrate with high performance, the composited metal/semiconductor heterostructures are usually considered [14,15]. In this heterostructure, both the electromagnetic mechanism (EM, for metal) and chemical mechanism (CM, for semiconductor) can be utilized, which firstly promises the high Raman sensitivity of SERS substrates. The EM originates from the surface plasmon resonance (SPR) effect [16,17], which is usually presented as intensive localized electric fields (LEFs) around metal nanoparticles (NPs). The LEFs are also called as ‘hot spots’ in many literatures [18], playing a main role in SERS enhancement in contrast with the CM [19]. Many reports have shown that the LEFs in the nanogaps between adjacent metal NPs are much stronger compared with those around the isolated NPs due to the interactions [20,21]. The metal nanoparticle cluster (NPC) has plenty of three-dimensional (3D) nanogaps, which can provide abundant and strong LEFs to enhance Raman signals. Besides, due to the large specific surface area, NPC can also adsorb amounts of probe molecules, which is highly suitable to build SERS substrate [22]. The CM is related to the charge transfer (CT) effect between the SERS substrate and the probe molecule, and many well-known semiconductors such as graphene, graphene oxide (GO), molybdenum disulfide, zinc oxide, etc. have been proved with well CM [23,24]. Among these materials, GO is a prominent ‘candidate’ because of its abundant active oxygen sites which can remarkably improve its electrostatic [23]. This feature endows GO excellent selective adsorption of molecules and stable Raman signals with perfect bio-compatibility, homogeneity and chemical stability. Thus, it is highly imperative to build the composite combined the metal NPCs with GO to improve Raman performance. Nowadays, although some studies have reported outstanding Raman performance of the composited metal NPC/GO structure [23,25,26], the cost performances of them are still low, in consideration of the cumbersome preparation process [27–29].

To solve this problem, a SERS substrate with structure of blackberry-like silver/graphene oxide NPC (Ag/GO NPC) is designed and prepared by a fast capillarity-assistant self-assembly technology in this paper. As shown in Fig. 1, it can achieve strong Raman signals only by mixing the Ag NPs, GO and probe molecules together and dropping it on the surface of a pre-processed CuO film. Benefitting from the slits in interlaced CuO nanowires (NWs) bundles, the capillary force can spontaneously shrink mixture together to form the Ag/GO NPCs without any additional assistance, which is vividly exhibited in Fig. 2(a). When the substrate is used to detect the malachite green (MG), a common abusive germicide in aquaculture, the LOD below 0.1 µg/L (3.48 × 10⁻¹⁰ mol/L) and EF of 2.64 × 10⁷ are obtained. As mentioned above, the abundant nanogaps in this Ag/GO NPC provide ample ‘hot spots’, finally turning into high SERS performance. Interestingly, this technology can be further used on polyethylene terephthalate (PET) film, forming a translucent flexible SERS substrate (Fig. 1(I)), which subsequently realizes an in situ detection of trace pollutant in water. In consideration of the facile preparation process, well SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), this substrate presents high cost performance and promising application prospect.

Fig. 1 Preparation process of the blackberry-like Ag/GO NPCs: I. preparation on flexible PET film; II. preparation on Cu foil.

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Fig. 2 (a) mechanism of the capillarity-assistant self-assembly technology; (b) SEM image of the CuO NWs; (c) close-up figure of (b); (d) SEM image of the Ag/GO NPCs on CuO NWs, and the inset in (d) is the photograph of the Ag/GO NPCs SERS substrate; (e) TEM image of the Ag/GO NPC; (f₁)-(f₄) EDS mappings of elements of C, Ag, O and Cu; (g) XRD pattern of the Ag/GO NPCs prepared on Cu foil.

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2. Theoretical and experimental details

2.1. FDTD simulations

In the FDTD simulations, the refractive index data of Ag and GO were obtained from Johnson and Christy and Ruoff et al. respectively [30,31]. To simulate the Ag/GO NPCs, seven hexagonal packing Ag NPs with size of 75 nm were adopted, and the environmental medium was filled with GO. The nanogaps between adjacent Ag NPs were set from 1 nm to 7 nm, and the wavenumbers of both the incident and monitoring lights were 532 nm, matching the light in experiment. Moreover, no particular molecule adsorption and CT effect between Ag NPs and MG molecules are involved in the simulations.

2.2. Materials

Copper foil (0.1 mm of thickness, 3N, $\geq$ 99.9%), silver nitrate (AR, $\geq$ 99.8%), malachite green (BS, $\geq$ 95%), acetone (AR, $\geq$ 99.5%), ethanol (AR, $\geq$ 95%), ethylene glycol (CP, $\geq$ 98%), sulphuric acid (AR, 95% - 98%) and antiformin (CP, Cl^- $\geq$ 5.2%) were all bought from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Monolayer graphene oxide (size ca. 50 nm - 200 nm) and polyethylene terephthalate film (0.1 mm of thickness) were respectively obtained from Nanjing XFNANO Materials Tech. Co. Ltd. and Guangdong Dongshun Plastics Co. Ltd. Polyvinylpyrrolidone (Mw = 55000, $\geq$ 99%) were purchased from Sigma-Aldrich (USA).

2.3. Preparation of CuO NWs

Preparation of CuO NWs with different morphologies had been detailedly discussed in our previous report [32], and in this paper we adopted the interlaced CuO NWs bundles, which can be obtained as followings: Firstly, clean the Cu foil (0.5 cm × 0.5 cm) successively in acetone, ethanol and deionized water for 15 min. Then, immerse it in the sulphuric acid (1 M) for 1 min to remove the surface oxides. After that, rinse the Cu foil with deionized water and ethanol, and dry it under a stream of nitrogen. Subsequently, dip the cleaned Cu foil in diluting antiformin solution (20% of volume fraction) for 1 min and wash it with the deionized water. At last, dry in the air for 20 min.

2.4. Preparation of Ag/GO NPCs

To obtain the Ag/GO NPCs, the suspension of Ag NPs and GO NFs is essential: Firstly, certain amount of GO NFs were added into 5 mL of the water following by a 10 h of ultrasonic vibration to make the GO well dispersed in the solution. Then, 100 µL of the pre-synthesis dispersion of Ag NPs [33,34] were mixed in the GO aqueous dispersion, with a further ultrasonic vibration for 30 min as well. Finally, 10 µL of this mixed suspension was dropped on the surface of a preprocessed Cu foil, and then dried naturally.

2.5. Raman measurement

All the Raman spectra in this paper were collected from the Raman spectrometer (Horiba HR Evolution 800) by exposing the substrates under laser of 532 nm with the excitation energy of 0.48 mW. The integration time was of 8 s, and the diffraction grid was of 1800 gr/mm. To ensure the accuracy, all the Raman data appeared in this paper are the average values obtained at five different points on same substrate. The error bars in figures were estimated as the measurement uncertainty (Δ_m) according to the equation below:

Δ_{m} = \sqrt{\frac{\sum_{i = 1}^{n} {(S_{i} - \bar{S})}^{2}}{n (n - 1)}}

where S_i is the measurement value,

\bar{S}

is average value, and n is number of measurements, equaling to five in this paper.

2.6. Enhancement factor

According to the standard equation of the EF:

EF = \frac{(\frac{I_{S E R S}}{N_{S E R S}})}{(\frac{I_{R S}}{N_{R S}})} = \frac{N_{R S}}{N_{S E R S}} \times \frac{I_{S E R S}}{I_{R S}}

where N_RS and N_SERS are respectively the numbers of probe molecules contributing to the inelastic scattering intensity, evaluated by SERS and normal Raman scattering measurements and I_SERS and I_RS are respectively the intensities of the surface-enhanced and normal Raman scattering. In our experiments, the molecules were detected by dropping a certain volume of mixed solution (10 µL) on the surface of corresponding substrate. Thus, the N_RS/N_SERS can be calculated as:

\frac{N_{R S}}{N_{S E R S}} = \frac{C_{R S} \times V_{R S} \times N_{A} \times (\frac{A_{R S}^{L a s e r}}{A_{R S}^{D r y}})}{C_{S E R S} \times V_{S E R S} \times N_{A} \times (\frac{A_{S E R S}^{L a s e r}}{A_{S E R S}^{D r y}})}

where C (C_RS and C_SERS) is the concentration of MG solution dropped on different Raman-active bases, V (V_RS and V_SERS) is the corresponding volume of the droplet, which equals to 10 µL in this paper. N_A is Avogadro’s constant, and A^Laser and A^Dry are area of laser spot and dried area on different substrate, respectively. In our experiments, the

A_{R S}^{D r y}

is almost equals to the

A_{S E R S}^{D r y}

, and the

A_{R S}^{L a s e r}

and the

A_{S E R S}^{L a s e r}

are almost the same. Thus, the EF can be further estimated as:

EF = \frac{I_{S E R S}}{I_{R S}} \times \frac{C_{R S}}{C_{S E R S}}

In this paper, the processed Cu foil with CuO NWs is selected as the reference substrate, and its corresponding Raman performance is shown in Fig. 4(a) (MG of 100 mg/L). Choose intensity of peaks of 1618 cm⁻¹, 1590 cm⁻¹ and 1178 cm⁻¹ as the calculated standard (Ag/GO NPCs, MG of 0.1 µg/L), the EFs can be obtained as ca. 2.75 × 10⁷, 2.16 × 10⁷ and 3.01 × 10⁷, respectively.

Moreover, according to the EM theory:

{EF}_{T} = {EF}_{Loc} \times {EF}_{Rad}

where EF_T is the theoretical EF, EF_Loc is the local field EF related to excitation of the Raman dipole, and EF_Rad is the radiation EF originating from Raman dipole emission [35,36]. Further, the EF_Loc and EF_Rad can be estimated as:

{EF}_{Loc} \propto {| \frac{E_{Loc} (ω_{0})}{E_{0} (ω_{0})} |}^{2}

{EF}_{Rad} \propto {| \frac{E_{Loc} (ω_{R})}{E_{0} (ω_{R})} |}^{2}

where E_Loc and E₀ are the localized electric field and incident electric field caused by LSPR and laser power respectively, and ω₀ and ω_R are incident frequency and Raman scattering frequency respectively. For low-frequency vibrational modes of adsorbed molecules, the ω₀ and ω_R are usually comparable, thus the EF_T can be further estimated as:

{EF}_{T} \propto {| \frac{E_{Loc}}{E_{0}} |}^{4}

The max E_Loc obtained in our simulation is ca. 55-times higher than E₀ (Fig. 4(f)), thus the EF_T can be calculated ca. 9.16 × 10⁶, which is slightly smaller than the EF by experiment. However, in consideration that the real SERS enhancement is brought by a synergistic effect of EM and CM, the EF_T is reasonable.

3. Results and discussions

3.1. Morphology and phase

To get the Ag/GO NPCs, the interlaced CuO NWs bundles should be firstly obtained by soaking Cu foil into NaClO diluent, described in detail in the Part 2 (Theoretical and experimental details).

As presented in the scanning electron microscopy (SEM) images (Figs. 2(b) and 2(c)), the CuO NWs bundles are pine-like nanoneedles, which are uniform and with micro-scale length and diameter of tens of nanometers. Particularly, the NWs in these bundles are interlaced each other, leading to lots of slits between the adjacent ones (marked in Fig. 2(c)). To further build the Ag/GO NPCs, the processed suspension of Ag NPs and GO nanoflakes (NFs) was dropped on the surface of these CuO NWs bundles. After that, the jam-packed Ag/GO NPCs formed at the top of the bundles, with the sizes of ca. 400 nm - 600 nm (Fig. 2(d)). According to the corresponding transmission electron microscopy (TEM) image (Fig. 2(e)), it can be observed that the adjacent Ag NPs in this structure are spaced by the GO NFs, and the intervals between them are from 1 nm to 10 nm. Moreover, the size of Ag NPs is ca. 70 nm-90 nm in this structure, which have both better far-field and near-field effects [18,25]. According to the absorption spectrum of the suspension of Ag NPs and GO NFs (Fig. 3(b)), the LSPR peak of the Ag/GO NPC is located at 475 nm, and a crest around 423 nm - 550 nm implies that there is a broad absorption region for this sturcture, well matching with the exciting light (λ = 532 nm) in subsequent Raman test. To test the purity of these composite materials, X-ray diffraction (XRD) was performed. As shown in Fig. 2(g), the peaks located at 38.3° and 38.7° correspond to the (111) planes of the cubic phase of Ag and the monoclinic phase of CuO respectively, and the peak around 11.6° originates from the (001) plane of GO. The peaks located at 43.5°, 50.6° and 74.3° are all related to the cubic phase of Cu, which are mainly brought by the Cu foil. No other impure peaks are observed in this pattern, indicating the high purity and crystallization of the SERS substrate. Figure 3(a) and Figs. 2(f₁)–2(f₄) are the energy dispersive spectrum (EDS) and the elements mappings of this substrate respectively. The sharp characteristic peaks and the uniform elements distributions of C, Ag Cu and O confirm the presence of GO NFs、Ag NPs and CuO NWs in this substrate, corresponding well with the results obtained by the XRD. Furthermore, Fig. 2(a) demonstrates the formation process of this Ag/GO NPC vividly: Initially, the water permeates into the slits formed by CuO NWs bundles after the dropping of the mixture (Ag NPs and GO NFs). Subsequently, the water volatilizes up along the NWs bundles, and meanwhile the capillary force brings the Ag NPs and GO NFs nearby the surfaces of the NWs bundles (Fig. 2(a: I)). Along with the volatilization, the Ag NPs are continually compressed by the capillary force, with GO NFs filling in the intervals between them. Finally, the jam-packed Ag/GO NPCs are formed on the top of the NWs bundles, seeming like a blackberry (Fig. 2(a: II)).

Fig. 3 (a) EDS of the SERS substrate with Ag/GO NPCs; (b) absorbance of the suspension of Ag NPs and GO NFs

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3.2. Raman performances

To test the Raman performance of this composited structure, the MG, one of most commonly used germicide in aquaculture was selected as the probe molecule. The Raman spectra of MG (100 mg/L) on different substrates (CuO NWs; Ag NPCs and Ag/GO NPCs) are firstly shown in Fig. 4(a), and the intensity of peaks of 1618 cm⁻¹, 1590 cm⁻¹ and 1178 cm⁻¹ in these spectra are further exhibited in Fig. 4(c). In these figures, no obvious peaks can be detected by the CuO NWs substrate. However, if the Ag NPs are mixed in the MG solution before the dropping (i.e. the Ag NPCs), a strong signal enhancement appears. This phenomenon is mainly caused by the synergistic effects of the LSPR and the CT between Ag and CuO, which had been testified in many researches [37,38]. Further mixing the GO NFs in the colloidal solution (Ag/GO NPCs), the Raman intensity almost increases by 1.5 times in contrast with that collected from Ag NPCs, mainly originating from the additional CM enhancement and adsorption capacity of the GO. The influence brought by GO on Raman intensity is also investigated in this paper, with the results shown in Figs. 4(b) and 4(d). It is observed that although GO plays an important role in the SERS enhancement of Ag/GO NPC, excessive or insufficient GO may be counterproductive. In our experiments, the optimal concentration of GO is ca. 6.25 × 10⁻⁴ g/L. The reasons for this phenomenon are manifold (Fig. 4(e)): On one hand, GO has abundant active oxygen sites which can remarkably improve its electrostatic (well CM), and on the other hand, the special two-dimension structure endows it high specific surface area which can make it adsorb more probe molecules. Therefore, after certain amounts of GO are mixed in the Ag NPCs, the Raman intensity is enhanced significantly. However, when more GO are added, the overmuch GO fills in the nanogaps, and makes these gaps wider. According to the finite-difference time-domain (FDTD) simulations, it can be found that when the nanogaps increase from 1 nm to 7 nm, the maximal intensity of the simulated electric field around Ag/GO NPCs decreases by 5 times as shown in Fig. 4(f). Therefore, although stronger CM and adsorption can be brought by more GO, the declined EM makes the SERS performance worse.

Fig. 4 (a) Raman spectra of MG (100 mg/L) on different substrates; (b) Raman spectra of MG (100 mg/L) on Ag/GO NPCs with different amounts of GO; (c) and (d) are the intensity changes of peak 1618 cm⁻¹, 1590 cm⁻¹ and 1178 cm⁻¹ in the spectra in (a) and (b) respectively; (e) the influence of GO on SERS enhancement; (f) simulated electric field distributions for Ag/GO NPCs with different nanogaps.

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To test the sensitivity of this substrate, Raman spectra of MG with different concentrations (from 100 µg/L to 0.1 µg/L) were detected, with results shown in Fig. 5(a). From these spectra, it can be observed that the LOD of MG collected from this substrate is ca. 0.1 µg/L (3.48 × 10⁻¹⁰ mol/L), and an estimated average EF of this substrate can be calculated as 2.64 × 10⁷ (more details can be found in the Part 2 (Theoretical and experimental details)), demonstrating the high Raman sensitivity of this substrate. It should be noted that this EF is an average value collected from the irradiation area, which is usually smaller than the real one by a single ‘Ag/GO NPC’. Even so, the SERS performance of this substrate is still outstanding, by contrast with the Raman sensitivity of other substrates (Table 1).

Fig. 5 (a) Raman spectra of MG on Ag/GO NPCs with different concentrations; (b) linear fitting for the intensity changes of peak 1618 cm⁻¹, 1590 cm⁻¹ and 1178 cm⁻¹; (c) the intensity changes of some characteristic peaks of MG tested on twenty-five different points on Ag/GO NPCs; (d) Raman mapping with area of 400 µm² for peak 1618 cm⁻¹; (e) Raman spectra of MG tested on different days in one week; (f) intensity changes of Raman spectra of MG on Ag/GO NPCs with ten different batches.

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Table 1. Comparisons of SERS performance of Ag/GO NPCs and other known substrates

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Moreover, the well linear relation of intensity of peak 1618 cm⁻¹ etc. with different concentrations under the log scale (Fig. 5(b)) also verifies this conclusion from the other side. Raman signals of twenty-five different positions on the substrate were further detected to test the uniformity, and the intensity changes of some characteristic peaks (from 1618 cm⁻¹ to 920 cm⁻¹) can be found as shown in Fig. 5(c). It’s observed that all the intensity fluctuates only in a narrow range, and the corresponding relative standard deviations (RSD) alter just from 8.95% to 11.62% as well, implying the well uniformity of this substrate. The further Raman mapping test (Fig. 5(d)) for peak 1618 cm⁻¹ also confirms the conclusion. To evaluate the stability of this structure, the Raman spectra of MG were tested day by day in one week (Fig. 5(e)). Benefitting from the cladding of the GO, which can retard the oxidation reaction of the inner Ag NPs and acts as a protective layer for this composited structure, no obvious variation are found between these spectra, confirming the well stablitiy of this substrate. Besides, SERS enhancements of the Ag/GO NPCs prapared by different batches behave similarly as well (Fig. 5(f)), demonstrating the fine repeatability of the preparation process. It shoud be mentioned that due to the low-cost materials (PET: 3.23 $/m², Cu foil: 107 $/m², Ag suspension: 22.39 $/ml and GO: 14.93 $/g, etc.) and facile preparation way, the cost price of this substrare is only ca. 1.2 $ (LOD of 3.48 × 10⁻¹⁰ mol/L for MG), which is much lower than many commercial SERS sensors e.g. SERS-450-Au by Topmembranes Technology Co., Ltd. (8.98 $, LOD of 10⁻⁶ mol/L for MG), RAM-SERS-Au-5 by Ocean Optics (13.6 $, LOD of 1.04 × 10⁻⁸ mol/L for MG), RAM-SERS-Ag-5 by Ocean Optics (13.2 $, LOD of 1.04 × 10⁻⁸ mol/L for MG), showing promising commercial applications.

3.3. In situ detection of trace hazardous chemicals

As mentioned above, the probe molecules are mixed with the Ag NPs and GO NFs together before dropping. Therefore, after the Ag/GO NPCs formed, these molecules are distributed throughout the interior of the NPCs, which can obtain well SERS enhancment from both the EM (Ag) and CM (GO). However, it is ambiguous that whether the Raman performance will decline once the molecules are added after the formation of the Ag/GO NPCs.

The cladding of GO can reduce the contaction between the inner structure and the outside gas molecules, which is effective to probe molecules as well. Figure 6(a) shows the Raman spectra comparison of MG obtained by different adding ways. However, the differences between these spectra are not obvious as anticipated. For example, the Raman intensity of MG (100 mg/L) obtained by mixing are only ca. 2-times higher than that by dropping. Moreover, it is interesting that the intensity differences shrink gradually along with the decrease of concentration. Figure 7 shows the changes of intensity ratios of some characteristic peaks between the two adding ways, from which it can be observed that intensity ratios almost equal to 1 when the concentration is lower than 1 mg/L. To figure out this phenomenon, the morphology of this composited structure is re-observed. Interestingly, it can be found that the outside GO cladding is slightly fragmentary (marked in Fig. 6(b)), which has some broken nanoholes on the surface, meaning the Ag/GO NPCs are not compeletely isolated from the outside. Therefore, some of the MG molecules can still permeate into the inner of the structure (Figs. 6(c)–6(e)). However, the cladding of GO has an adverse impact more or less on the contaction between probe molecules and internal Ag and GO, especially for MG with high concentration. Thus, the Raman intensity of MG obtained by mixture is weaker than that by dropping. Nevertheless, when the concentration is below a certain point, the obstacle brought by GO is no longer the key factor for SERS enhancement, in which case the few probe molecules limit the increase of Raman intensity. Thus, there are almost no differences between the two adding ways. In further in situ detection of trace hazardous chemicals, the interaction between the probe molecules and substrate is similar to that in the dropping test, thus the above experimental results guarantee the operability of the in situ collection of Raman signals.

Fig. 6 (a) Raman spectra of MG obtained by two different adding ways; (b) morphology details of the Ag/GO NPCs; (c)-(e) blocking effect for probe molecules by GO under two different adding ways.

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Fig. 7 Changes of intensity ratios of some characteristic peaks between the two different adding ways.

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Through a similar method metioned above (more details can be found in the Part 2 (Theoretical and experimental details)), a translucent flexible SERS substrate of Ag/GO NPCs on PET film is obtained (Fig. 8(a)). Although the macroscopical morphology of this substrate is different from the inset in Fig. 2(d), the SEM images (Figs. 9(a) and 9(b)) indicate that the microstructures of them are the same. This fact can also be reflected by the Raman spectra from the other hand (Fig. 9(c)), where the Raman intensity of MG are as high as those in Fig. 5(a). Besides, benefitting from the translucence feature of the PET film, one can detect the Raman signals from both side of the substrate, like Figs. 9(c) and 9(d) shows. The signals obtained from the backface is slightly weaker than that detected from the front (Fig. 9(e)), which mainly originates from the lights scattering of the backface of the PET film. Even so, this phonomenon still indicates the further feasibility of the in situ detection by this substrate. In the subsequent test, the prepared PET SERS substrate was inverted on the surface of the MG solution as shown in Fig. 8(c), and due to the lightness and flexibility of the PET, this substrate can easily float on the surface of water. The corresponding Raman spectra detected under this situation (Fig. 8(b)) are exhibited in Fig. 8(e), from which all the characteristic peaks can be clearly observed, indicating a well in situ detectability of this substrate. Linearity of the intensity changes of the characteristic peaks with different concentrations can also be observed (Fig. 9(f)), with a average correlation coefficient (R²) of 0.943. For further testing the practicality of this substrate, the fishpond water was taken and in situ detected in our experiments. Figure 8(f) shows the Raman spectrum of this sample, in which the intensity of some peaks such as 1618 cm⁻¹, 1590 cm⁻¹, 1780 cm⁻¹, etc. is weakly observed, demonstrating the existence of the MG. According to the linearity displayed in Fig. 9(f), the concentration can be estimated ca. 0.3 µg/L (1.04 × 10⁻⁹ mol/L). Figure 8(d) shows the schematic of this in situ detection.

Fig. 8 (a) photograph of the translucent flexible SERS substrate of Ag/GO NPCs; (b) and (c) in situ Raman detection of MG; (d) schematic of the in situ detection; (e) Raman spectra of MG with different concentrations by in situ detection; (f) Raman spectra of fishpond water.

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Fig. 9 (a) and (b) are the SEM images of the CuO NWs and Ag/GO NPCs respectively, both of which are prepared on PET film; (c) front detection and (d) back detection of Raman spectra of MG on translucent flexible SERS substrate of Ag/GO NPCs with different concentrations (from 100 µg/L to 0.1 µg/L); (e) the intensity changes (1618 cm⁻¹) under different concentration by the two detection ways; (f) linear fitting for the intensity changes of peak 1618 cm⁻¹, 1590 cm⁻¹ and 1178 cm⁻¹ (in situ detection by the PET).

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4. Conclusions

In brief, to obtain a cost-effective SERS substrate with high performance and realize an efficient in situ detection of trace hazardous chemicals, a blackberry-like Ag/GO NPC SERS substrate is designed and prepared through a fast capillarity-assistant self-assembly technology in this paper. On account of the abundant ‘hot spots’ and active oxygen sites brought by the Ag/GO NPCs, this substrate exhibits well Raman performances for MG, with LOD below 0.1 µg/L (3.48 × 10⁻¹⁰ mol/L). Constructing this structure on PET film, a translucent flexible SERS substrate can be obtained in our experiment, which subsequently realizes an efficient in situ detection of the trace MG in fishpond water. Further analysis is taken both on the formation process and SERS enhancement mechanism for this structure, and the FDTD simulations are also introduced to prove our hypotheses. Considering the facile preparation process, well SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), the blackberry-like Ag/GO NPC SERS substrate presents high cost performance and promising application prospect.

Funding

National Natural Science Foundation of China (21701102); Natural Science Foundation of Shandong Province (ZR2017BA018); Hong Kong Scholars Program (XJ2018025), Shandong Provincial Science and Technology Project (J17KZ002); China Postdoctoral Science Foundation (2017M612322, 2017T100511, and 2016M600550).

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Samples	LOD for MG	EF	In situ detection
Ag/GO NPCs (this paper)	3.48 × 10⁻¹⁰ mol/L	2.64 × 10⁷	Well
Ag NPs & agar/polyacrylonitrile nanofibers [39]	1.00 × 10⁻⁷ mol/L	3.10 × 10⁵	Well
Ag NPs/TiO₂ Nanorod [40]	1.00 × 10⁻¹² mol/L	4.36 × 10⁵	None
nano-imprinted Ag array film [41]	1.00 × 10⁻⁸ mol/L	2.00 × 10⁶	None
Ag nanoclusters on ZnO nanodome array [42]	1.00 × 10⁻¹⁷ mol/L	1.00 × 10⁶	None
Ag NPs coated paper [43]	1.00 × 10⁻⁸ mol/L	2.20 × 10⁷	None

In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

Abstract

1. Introduction

2. Theoretical and experimental details

2.1. FDTD simulations

2.2. Materials

2.3. Preparation of CuO NWs

2.4. Preparation of Ag/GO NPCs

2.5. Raman measurement

2.6. Enhancement factor

3. Results and discussions

3.1. Morphology and phase

3.2. Raman performances

3.3. In situ detection of trace hazardous chemicals

4. Conclusions

Funding

References

Cited By

Figures (9)

Tables (1)

Equations (8)

Optics Express