Elsevier

Applied Surface Science

Volume 529, 1 November 2020, 147121
Applied Surface Science

Full Length Article
Improved lateral flow strip based on hydrophilic−hydrophobic SERS substrate for ultra−sensitive and quantitative immunoassay

https://doi.org/10.1016/j.apsusc.2020.147121Get rights and content

Highlights

  • A hydrophilic−hydrophobic Ag/PDMS SERS substrate was developed.

  • Lateral flow strip was improved by utilizing the hydrophilic−hydrophobic strategy.

  • Quantitative SERS analysis of Ferritin was realized by using the intrinsic Raman signal of PDMS.

Abstract

As a convenience and robust point−of−care diagnostics technique for biomarkers, surface−enhanced Raman scattering (SERS)−based lateral flow immunoassay (LFIA) has attracted attentive attentions. In traditional paper−based LFIA, its hydrophilic surface with the so−called coffee−ring effect will inhibit the sensitivity and quantitative ability, which made the results less satisfying. Here, an attractive hydrophilic−hydrophobic polymer strip with Raman internal standard (IS) was developed by magnetron sputtering hydrophilic Ag nanoparticles (NPs) onto the specific area of the hydrophobic polydimethylsiloxane (PDMS). The target analytes can be enriched on the test and control lines formed by the hydrophilic Ag regions, which significantly improved the adaptability of SERS−based LFIA strip in the ultra−sensitive and quantitative monitoring of trace Ferritin (FER). Typically, assisted by raspberry−like Au NPs as immunoprobes, such hydrophilic−hydrophobic LFIA strip could achieve a limit of detection for FER as low as 0.41 pg/mL. Moreover, calibrated linear relationship with R2 values of 0.992 in the analysis was successfully demonstrated by rationally using the Raman signals of PDMS as a reference. Such an improved LFIA platform may give another promising technique for the trustworthy monitoring and treatment of cancer.

Introduction

Surface−enhanced Raman scattering (SERS) technique with promising fingerprint recognition and ultra−high sensitivity has been confirmed as a powerful protocal in food safety, chemical analysis, medical detection, and pollutant monitoring [1], [2], [3], [4], [5]. In fact, a possible significant progress in the practical application of SERS technique is mainly determined by the rational synthesis of substrates based on noble metal nanoparticles (NPs) with unique electromagnetic enhancement ability [6], [7], [8], [9]. In the field of biomedical monitoring, SERS−based lateral flow immunoassay (LFIA), composed by several pieces of assembled paper−based material on a plastic backboard, has become one of the exhaustively used protocols for point−of−care testing (POCT) diagnostics due to its low cost, promising convenience, rapid detection, and user−friendly format [10], [11], [12], [13]. In a typical test based on sandwich immunostructure, a drop of immunoprobes is dropped onto the strip and it moves through the membrane by capillary force to reach the test or control zone labelled with target antigens or bare antibodies. However, the major disadvantage of this assay is its limited accumulative and quantitative abilities, which make these LFIA biosensors less attractive.

Besides the similar flexibility, portability, and simplicity to paper−based supports, the most superiority of polydimethylsiloxane (PDMS) for serving as the ideal matrix of plasmonic NPs lies in not only its intrinsic Raman peaks but also its naturally hydrophobic surface. In fact, the intrinsic Raman peaks could be utilized as internal standard (IS) to effectively realize quantitative test and the hydrophobic surface can prevent the random spreading of analyte solution to fully concentrate them [14], [15], [16], [17], [18] Particularly, a novel hydrophilic−hydrophobic assembly substrate has been designed to facilitate the uniform distribution of analytes within the hydrophilic area surrounded by hydrophobic zone, which can hinder coffee−ring effect and enable more accurate detection [19]. Although some SERS substrates using PDMS film as support have been developed [14], [20], it should be noted that there is nearly no report about versatile LFIA with both Raman IS and enrichment ability based on the PDMS. If such a novel platform was utilized in the practical biomedical assay, highly sensitive and reliable quantitative results could be definitely imagined.

In this communication, a novel PDMS−based LFIA was proposed for the ultra−sensitive and quantitative SERS analysis of Ferritin (FER), a typical kind of tumor markers that play a crucial role in the efficient detection of liver cancer (Scheme 1). In this innovative LFIA, the microdroplets of analytes could flow smoothly in the hydrophobic surface of PDMS and then captured and enriched by the test line of hydrophilic Ag surface. Without the coffee−ring effect, such a hydrophilic−hydrophobic SERS platform would achieve an on−site trace FER detection with a pretty good limit of detection (LOD, 0.41 pg/mL), assisted by the raspberry−like Au NPs (RANPs) as immunoprobes. Moreover, the corresponding linear fittings were steadily improved by rationally using the Raman signals of PDMS as a reference. We believe such a novel PDMS−based LFIA would pave a new way for ultra−sensitive and quantitative analysis in the POCT setting.

Section snippets

Preparation of hydrophilic−hydrophobic Ag/PDMS immunosubstrate

Ag NPs was selectively coated onto the specific areas of PDMS matrix assisted by homemade mask to create three hydrophilic regions using a typical magnetron sputtering apparatus (0.4 Pa, 40 W). In order to prepare the immunosubstrate, the captured anti−FER in PBS solution (20 μL, 0.2 mg/mL) was modified onto the Ag/PDMS through incubation (4 °C, 12 h). BSA was then used to cover the extra bare sites of the matrix. After being modified by captured anti−FER and target FER, one of the hydrophilic

Results and discussion

It is well accepted that the SERS performance of noble metal depends on its configuration parameters such as surface morphology, roughness, and homogeneity [21], [22]. In this regard, the optimal decoration amount of Ag for obtaining appropriate Raman enhancement effect was firstly investigated after the formation of hydrophilic−hydrophobic Ag/PDMS substrates. Fig. 1a−d illustrate the morphologies of Ag/PDMS substrates prepared under different sputtering time (3 to 12 s). As can be identified,

Conclusion

In summary, with the aim to improve the sensitivity and quantitative degree of SERS−based detection of FER, we designed a novel LFIA mediated by a sophisticated hydrophilic−hydrophobic Ag/PDMS substrate. Pretty high SERS intensity with EF of 1.93 × 106 was facilitated by such a hydrophilic−hydrophobic strategy. More meaningfully, analytes during rolling could be stopped in the hydrophilic lines (Ag) surrounded by hydrophobic PDMS regions for more efficient anchoring. Supported by the

CRediT authorship contribution statement

Yi Ma: Investigation, Methodology, Writing - original draft. Hongmei Liu: Investigation. Ying Chen: Investigation. Chenjie Gu: Investigation. Guodong Wei: Investigation. Tao Jiang: Writing - review & editing, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Natural Science Foundation of Zhejiang Province (Grant No. LY19F050002), Natural Science Foundation of Ningbo (Grant No. 2018A610316), and K.C. Wong Magna Fund in Ningbo University, China.

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