Fast and efficient deposition of broad range of analytes on substrates for surface enhanced Raman spectroscopy

https://doi.org/10.1016/j.bios.2020.112124Get rights and content

Highlights

  • Fast and efficient deposition of analytes in electric field was developed.

  • Technique was applied to improve surface-enhanced Raman spectroscopy.

  • Method allows for reduction of time of deposition and sample volume.

  • Wide range of analytes was deposited (proteins, dyes, drugs, biomarkers).

  • Deposition parameters and electrophoretic mobility of analytes were correlated.

Abstract

The majority of analytical chemistry methods requires presence of target molecules directly at a sensing surface. Diffusion of analyte from the bulk towards the sensing layer is random and might be extremely lengthy, especially in case of low concentration of molecules to be detected. Thus, even the most sensitive transducer and the most selective sensing layer are limited by the efficiency of deposition of molecules on sensing surfaces. However, rapid development of new sensing technologies is rarely accompanied by new protocols for analyte deposition. To bridge this gap, we propose a method for fast and efficient deposition of variety of molecules (e.g. proteins, dyes, drugs, biomarkers, amino acids) based on application of the alternating electric field. We show the dependence between frequency of the applied electric field, the intensity of the surface enhanced Raman spectroscopy (SERS) signal and the mobility of the studied analyte. Such correlation allows for a priori selection of parameters for any desired compound without additional optimization. Thanks to the application of the electric field, we improve SERS technique by decrease of time of deposition from 20 h to 5 min, and, at the same time, reduction of the required sample volume from 2 ml to 50 μl. Our method might be paired with number of analytical methods, as it allows for deposition of molecules on any conductive surface, or a conductive surface covered with dielectric layer.

Introduction

The ultimate goal of all analytical techniques is to selectively detect single, non-labeled molecules in the fastest, easiest, repeatable and inexpensive way. Typical sensor consists of the sensing layer and the transducer. Some of the most commonly used transducers are: quartz crystal microbalance (Cooper and Singleton, 2007), optical fibers (El-Sherif et al., 2007), electrochemical cell (Wu et al., 2015), spectroscopic methods including surface enhanced luminescence (Wokaun et al., 1983) or fluorescence (Fort and Grésillon, 2008), surface plasmon resonance (Homola et al., 1999), localized surface plasmon resonance (Mayer and Hafner, 2011), surface enhanced Raman scattering (Campion and Kambhampati, 1998), and more. All above-mentioned methods are similar in one important aspect: they require proximity of an analyte to be detected and the sensing surface. Therefore, precise and well-controlled deposition of the analyte on the sensing surface is one of the most important aspects of many detection processes. Surprisingly, development of new methods of efficient deposition of analytes is often omitted and thus still poorly addressed. Here, we present versatile method of deposition of variety of analytes on the surface by application of the electric field, which we use to improve performance of surface enhanced Raman spectroscopy (SERS).

SERS utilizes localized surface plasmon resonance (LSPR) of metal surfaces to obtain ultrahigh enhancement of Raman scattering of molecules (Albrecht and Creighton, 1977; Fleischmann et al., 1974; Jeanmaire and Van Duyne, 1977; Moskovits, 1979). The technique allows for multiplex detection of non-labeled molecules. SERS offers signal enhancement comparing to classical Raman ranging from 102 to 106 and with proper design even from 108 to 1015 (Nie and Emory, 1997). Moreover, SERS provides the ultimate sensitivity, i.e., detection of a single molecule (Nie and Emory, 1997). Many applications of SERS were described in literature, ranging from archeology (Bruni et al., 2010), analysis of works of art (Centeno and Shamir, 2008), forensic science (Muehlethaler et al., 2016), detection of bioterrorist threats (Hakonen et al., 2015), diagnostics (Alvarez-Puebla and Liz-Marzán, 2010), bacteria and virus detection (Jarvis and Goodacre, 2008; Shanmukh et al., 2006) etc. The design of efficient and versatile SERS substrate is currently a hot topic among material scientists (Lee et al., 2019; Prakash et al., 2019; Zeng et al., 2019). Unfortunately, despite such wide range of developed applications, there are still barely any practical implementations of SERS outside academia.

One of the main reasons for such relatively scarce use of SERS is long time of analysis required for SERS detection. Standard protocol assumes immersion of SERS substrate in the sample for up to 20 h, which is too long for many practical cases (de Almeida et al., 2019; Prokopec et al., 2011). Approaches aiming to decrease time of sample preparation, such as drop casting and drying are prone to homogeneity issues, for instance generate coffee ring effect. This might also result in majority of hotspots (regions of SERS substrates providing the highest enhancement) remaining unoccupied. To obtain dense and homogenous surface coverages and strong SERS signal, long times of deposition and/or high concentrations of analyte are usually required. However, such conditions are in stark contrast with goal of the analytical chemistry. Thus, efforts to create the most efficient SERS substrates need to be supplemented with the development of methods of analyte application on such surfaces. Unfortunately, until now, very little attention has been paid to this problem.

We aimed to bridge this gap by using external electric field. The method is versatile as charges (formal, partial and delocalized) are ubiquitous in nature. Main limitation of such approach is that it requires analyte to possess charge. However, as majority of molecules is charged, especially in aqueous solutions, this should not greatly limit future applications of developed technology. Another requirement of our method is that sensing surface should be conductive or should be coated on top of conductive layer. This is fulfilled for number of analytical techniques. Here, we demonstrate the potential of the method on the example of SERS. Although some SERS substrates could have weaker conductivity (i.e., nanoparticles sparsely distributed on paper or quartz support), many of them have dense metal nanostructures or conductive support (i.e., indium tin oxide). The electric field was previously used to modulate SERS spectra (Sriram et al., 2012) and to improve selectivity of detection of various molecules (Almohammed et al., 2019; Walia et al., 2015). The most exploited approach combining SERS and the electric field is called electric field-induced SERS. It is used to enhance and modulate the signal coming from the molecules that are already present at the SERS surface (e.g. by changing their conformation at the surface).

There were few examples of improved deposition of analyte on SERS substrates by the utilization of the electric field. One of them is demonstrated in the work in which constant electric field was applied between parallel electrodes for selective deposition of few types of dyes (Lacharmoise et al., 2009). Applied electric field increased both selectivity and sensitivity of detection. Very similar approach was later proven to be useful also for adenine, serotonin, Congo red dye or Rhodamine 6G (Cho et al., 2009; Park et al., 2015). Combination of SERS and the electric field was also used in lab-on-a-chip devices. For instance, chip with microwells enabled detection of viral DNA (Huh et al., 2009). Potential applied to bottom of each well caused concentration of detected particles. Another example is photonic-plasmonic hybrid type Raman nanosensor integrated with electrokinetic manipulation, which allowed for detection of biologically and environmentally relevant molecules, including Nile blue, adenine, and melamine (Liu et al., 2017). Besides direct electrophoresis, also other electrokinetic mechanisms were used for deposition. For example, dielectrophoresis was successfully employed to selectively concentrate bacteria from blood on SERS surfaces (Cheng et al., 2013).

All abovementioned examples of utilization of the electric field for SERS share at least three major drawbacks, that hinders their broad practical implementation. (i) Deposition and SERS analysis were performed simultaneously in the same device. Such approach, although useful in some cases (e.g. basic research), is highly limiting in practical applications as it requires modification of SERS setups. (ii) Both electrodes were directly immersed in the solution, which could cause Faradaic current flow and electrochemical reactions. Although they can be beneficial in some cases (Sivanesan et al., 2014), if uncontrolled, they can influence the composition of the sample, or even cause decomposition of both nanostructures and deposited molecules. (iii) All described approaches were focused on one or just a few analytes. To date, there is no comprehensive analysis of deposition of variety of compounds in the electric field. Here, we present the method of deposition of the analyte on the SERS surface that omits all of abovementioned issues. (a) Developed setup allows for 5-min deposition of broad range of analytes that was performed in the separate device as independent step before SERS analysis. (b) One of the electrodes is isolated, thus no Faradaic currents flow through the system. (c) Our method is based on application of the alternating electric field. Frequency of the applied voltage can be adjusted to fit the characteristics (mainly mobility) of the studied analyte.

Section snippets

Materials

Chemicals were purchased from Sigma Aldrich (USA) and were used without further purification. Designer drugs: 3′,4′-tetramethylene-α-pyrrolidinovalerophenone (denoted as TH-PVP) and α-pyrrolidinopentiothiophenone (denoted as α-PVT) were obtained as 10−2 M solution in methanol from Research Institute of Central Forensic Laboratory of the Police (Warsaw, Poland). N,N,N-trimethylbut-3-yn-1-aminium bromide was synthetized and purified as described in the section below. Para-mercaptobenzoic acid

Results and discussion

The utilized deposition cell was in fact a capacitor filled with the electrolyte. In such system, immediately after application of the voltage, the charged species present in the solution move accordingly and create the electrical double layers (EDLs). As a result, the effective electric field in the solution is screened by EDLs and eventually drops to zero. When this process is completed, no electric field is present in the bulk and thus no additional movement of the molecules in the electric

Conclusions

In case of many analytical techniques effective detection is usually governed by two distinct processes: effective delivery of the analyte to the sensing surface and generation of signal from adsorbed analytes. The second issue is broadly addressed, as new sensors and sensing techniques are reported daily. This effort has to be complemented with development of methods that enable effective deposition of analyte on the sensing surface. This is crucial to overcome challenges of modern analytical

CRediT authorship contribution statement

Łukasz Richter: Methodology, Investigation, Validation, Formal analysis, Writing - original draft, Visualization, Project administration, Funding acquisition. Paweł Albrycht: Investigation, Validation. Monika Księżopolska-Gocalska: Investigation. Ewa Poboży: Investigation. Robert Bachliński: Resources. Volodymyr Sashuk: Resources. Jan Paczesny: Writing - review & editing, Supervision, Funding acquisition. Robert Hołyst: Conceptualization, Writing - review & editing, Supervision, Funding

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

Ł.R. acknowledges the support of the National Science Centre, Poland, within the grant Preludium UMO-2017/27/N/ST4/02353. The work of R.H. was supported by the National Science Centre, Poland, within the grant Maestro UMO-2016/22/A/ST4/00017. J.P. was supported by the National Science Centre, Poland, within the grant Sonata Bis 2017/26/E/ST4/00041. The authors would like to thank the SERSitive for providing high quality SERS substrates. Authors would like to acknowledge Agata Tereszkiewicz for

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