Flexible superhydrophobic SERS substrates fabricated by in situ reduction of Ag on femtosecond laser-written hierarchical surfaces
Introduction
Ever since the early observations of enhancement of Raman scattering signals upon roughening of a silver electrode, the surface enhanced Raman scattering (SERS) spectroscopy has become an extensively pursued tool due to its high intrinsic sensitivity and unique capability to spectroscopically “fingerprint” an analyte molecule [[1], [2], [3], [4]]. The potential of this technique has been successfully utilized in diverse areas, including photovoltaics, photocatalysis, molecular sensing, single cell spectroscopy, and cancer detection [[5], [6], [7]]. The enhancement in Raman signals that ensue upon the use of SERS substrates results from localized electric field enhancement that occurs when a resonance condition is attained between the incident electromagnetic field and collective oscillations of conduction electrons within the metallic substrate. As compared to enhancement that may be accomplished using chemical techniques, the fourth power dependent electromagnetic field contribution is what underpins the considerably larger SERS signal enhancement. Several orders of enhancement in the magnitude of Raman signals is now attained by placing an analyte in the gap between two or more metal structures, creating the so-called “plasmonic hot spots” [8,9].
Of the many methods that have been adopted to fabricate SERS substrates, ion-beam lithography and electron beam lithography prove to be of considerable utility [10,11]. However, these techniques are expensive to implement and suffer from low throughput. Contemporary techniques like nanosphere lithography (NSL), metal film over nanosphere (MFON), and shadow overlap of ion-beam lithography (SOISL) rely on the fabrication of masks by self-assembly of nanospheres on top of a base substrate followed by metal evaporation [[12], [13], [14]]. In parallel, direct self-assembly of metallic nanostructures has also been employed to fabricate SERS substrates [15,16]. However, the spatial uniformity and size of crystalline structure rely primarily on the self-assembly process. In contrast, SERS substrates are fabricated by coating silver or gold nanoparticles on a rough surface, thereby offering some degree of control over the process [17,18]. However, photoluminescence of the templates as well as of noble metallic films upon laser excitation limits the efficiency with which signals can be collected; this affects the information on the molecular vibrations of fluorescent molecules. Of late, fabrication of SERS substrates by biomimicking micro- or nano-scale features -on naturally occurring surfaces, like plant leaves and butterfly wings, has been reported [[19], [20], [21]]. These biomimicked substrates are found to exhibit tailored wettability which facilitates the concentration enrichment of analyte molecules, thereby improving the detection limit to ultra-low concentrations (10−18 M of Rhodamine 6G) [22]. Recently, laser direct writing (LDW) has emerged as an effective tool to biomimick naturally occurring surfaces by fabricating micro- and nano-scale structures on and within the surfaces; [[23], [24], [25], [26]] LDW offers significant advantages, such as a totally mask-less, green (chemical-free) approach, high throughput over a large area, and a high degree of controllability via the use of appropriate laser parameters. LDW is now beginning to be employed to induce SERS activity on diverse substrates including flexible substrates via in situ reduction of plasmonic particles on the surfaces. Moreover, the use of a femtosecond laser to carry out LDW produces little or no thermal effects, facilitates the writing of flexible SERS substrates on scaffolds like polymers, paper, and carbon fibers in a cost-effective fashion [[27], [28], [29], [30]].
Recent efforts in improving the lower limit of SERS detection of an analyte molecule have mainly focused on enriching the analyte molecule on a superhydrophobic surface comprising micro/nanoscale structures [[31], [32], [33], [34]]. Efforts using micro pillars on a regular silicon lattice covered with silver nanograin aggregate revealed a sensitivity to the concentration of Rhodamine 6G as low as 10−18 M [35]. The role of hydrophobicity on the improved limit of detection (LOD) was proved by a comparative study on superhydrophobic zinc oxide nanorods coated with silver nanoparticles, for stearic acid [36]. Inspired by naturally occurring superhydrophobic surfaces, such as a lotus leaf, butterfly wings, a rose petal, or rice leaves, laser-written structures are proving to be effective in controlling the wettability of materials like quartz, polymers, and silicon [[37], [38], [39]]. Surface texturing using femtosecond lasers results in a localized re-solidification of the laser-irradiated surface following laser ablation with minimum thermal effects; this locally alters the surface roughness with the extent of localization that is achieved via a controlled focusing of the incident laser beam. The surface roughness modification results in the formation of either the Wenzel state or the Cassie-Baxter state and makes the sample either more hydrophilic or hydrophobic/superhydrophobic [40,41]. Recently, sputtering of Ag atoms onto the hierarchical structures fabricated on a silicon surface via femtosecond laser patterning has been found to exhibit SERS activity with a LOD value as low as ∼10−15 M (femtomolar) for Rhodamine 6G (Rh 6G) [42]. The synergic effect of superhydrophobicity (fabricated via LDW) and plasmonic property of in situ reduced metallic nanoparticles may provide efficient substrates for SERS detection.
We present here the replication of femtosecond laser direct written grid patterns on the polymer, polymethylmethacrylate (PMMA) onto a flexible polymer (polydimethylsiloxane – PDMS) followed by in situ reduction of silver nitrate solution using the flexible polymer, PDMS, itself. Such a master-slave soft lithographic approach precludes the repeated usage of femtosecond laser while, concomitantly, the direct reduction of the salt by the PDMS results in the creation of uniform distribution of metallic particle on the surface. In addition to exploring the water spreading behavior (in terms of apparent contact angle and CAH) on the master as well as slave structures, we investigated the SERS efficiency of the metal reduced slave substrates for Rh 6G and methyl orange. The superhydrophobic metal reduced slave structures (rose petal substrate) is found to provide a LOD of 8 femtomolar for Rh6G and ∼32 nM for methyl orange with an enhancement factor >109. Our protocol is a cost-effective, general method that provides a simple, stable, flexible superhydrophobic SERS platform that can detect down to femtomolar levels using a simple fiber based excitation-collection geometry. Our approach is certainly amenable to offer trace-level molecular sensing on a platform that can be made portable.
Section snippets
Femtosecond laser direct writing (LDW)
Patterning of master structures on PMMA (obtained from Schönig + Domes, Germany) was performed using a Ti:Sapphire femtosecond oscillator (Femtolasers, Austria) which sends out a train of pulses of 50 fs duration at a repetition rate of 5.1 MHz (center wavelength: 800 nm; pulse energy: 200 nJ). A spot size of ∼4 μm was achieved using a microscope objective (10X, 0.25 N.A). Keeping the spot size fixed, the incident fluence on the sample was regulated using a combination of half wave plate and
Results and discussion
The direct femtosecond laser patterning on an intrinsically hydrophilic substrate, PMMA (CA ∼ 68 ± 2°), is found to increase the apparent contact angle with laser fluence (Fig. 1a), with a concomitant increase in CAH (Fig. 1b) along both perpendicular (with respect to X-axis) as well as parallel directions. We chose laser fluence in the range 262 mJ/cm2 to 734 mJ/cm2, well above the laser ablation threshold of PMMA (∼98 mJ/cm2) [45]. The atomic force microscope (AFM) image analysis of the
Conclusions
The replication of hierarchical structures fabricated on the surface of polymethylmethacrylate polymer via femtosecond laser patterning onto polydimethylsiloxane surface provides a cost-effective route to prepare flexible superhydrophobic surfaces with the lotus effect. Reduction of silver nanoparticles on the flexible hierarchically structured polydimethylsiloxane surface combines the advantage of superhydrophobicity and nanoplasmonics and provides a useful platform to mimic rose petal
Declaration of interest
Authors declare no competing financial interest.
Acknowledgements
We gratefully acknowledge financial support from the joint Manipal Academy of Higher Education and FIST program of the Government of India (SR/FST/PSI-174/2012). SDG acknowledges Manipal Academy of Higher Education for the Dr. TMA Pai Endowment Chair in Applied Nanosciences. JEG acknowledges Manipal Academy of Higher Education for financial support via the Dr. T. M. A. Pai Scholarship. We also acknowledge support for the femtosecond laser facility at Manipal Academy of Higher Education via the
Jijo Easo George is currently a Ph.D. student at Department of Atomic and Molecular Physics, Manipal Academy of Higher Education under Dr. T M A Pai Ph. D Scholarship Scheme. His research interest is tailoring of the surface wettability via physical chemical and electrical methods for diverse applications.
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Jijo Easo George is currently a Ph.D. student at Department of Atomic and Molecular Physics, Manipal Academy of Higher Education under Dr. T M A Pai Ph. D Scholarship Scheme. His research interest is tailoring of the surface wettability via physical chemical and electrical methods for diverse applications.
Dr. Unnikrishnan V. K is an Associate Professor from Department of Atomic and Molecular Physics, MAHE, Manipal. He obtained his Masters (Laser Physics) and MPhil degrees in Physics from Pondicherry University and PhD from Manipal University. His research interests include laser spectroscopic instrumentation and biomedical applications of laser. He also has nearly 30 publications in national/international journals and is a peer reviewer of many reputed journals.
Dr. Deepak Mathur obtained a Bachelors degree in Electronics from University of London and Ph.D from Birkbeck College, University of London. Presently, he is a JC Bose National Fellow at Department of Atomic and Molecular Physics, MAHE. Dr. Mathur is a recipient of the Bhatnagar Prize and Eminent Mass Spectrometrist Prize. He has been a Royal Society Guest Fellow at Oxford University and was awarded Fulton Fellowship by the Association of Commonwealth Universities. He was also the European Union’s Erasmus Mundus Scholar in Optical Science and Technology. He is a Fellow of Indian Academy of Sciences, Indian National Science Academy, and the World Academy of Sciences. He is an author of more than 320 journal articles.
Dr. Santhosh Chidangil received his Ph.D in Physics from Banaras Hindu University (India). Presently, he is a Professor and Head of the Department of Atomic and Molecular Physics at Manipal Academy of Higher Education. He was a senior associate of International Centre for Theoretical Physics (Italy) and recipient of Dr. T. M. A. Pai Endowment Chair in Biophotonics of Manipal University. His current research interests are atomic and molecular Spectroscopy (fluorescence, time-resolved fluorescence, Raman Spectroscopy, optical tweezers combined with Raman Spectroscopy, laser induced breakdown spectroscopy), proteomics and nanophotonics. He is an author of more than 110 journal articles.
Dr. Sajan D. George received his Ph.D. in Photonics from Cochin University of Science and Technology (India). He worked as a scientific colleague at Katholikek University of Leuven (Belgium), Leibniz University of Hannover (Germany), and Technical University of Darmstadt (Germany). In 2012, he moved to Manipal Academy of Higher Education as an Associate Professor and presently he is a Professor and Coordinator of the Centre for Applied Nanosciences. He is a recipient of Dr. T. M. A. Pai Endowment Chair in Applied Nanosciences and Editor of Scientific Reports, Nature. His research interests include micro/optofluidics, photothermal methods, nano-biophotonics, biomedical applications of laser, fluorophore-nanoparticle interactions. He is an author of more than 60 journal articles.