Nanofabrication of Polymer Nanotube-Based SERS Substrates and Subsequent Analytical Detection of Organic Molecules

Abstract

Based on the available literatures, the study seeks to provide an up-to-date literature anchored in principal of Surface Enhanced Raman Spectroscopy, which entails Electromagnetic Enhancement (EM), localised surface plasmons (LSP), localized surface Plasmon resonance, hot spot, and Chemical Enhancement (CE).

 

1.0 Introduction

The intrinsic diminutive Raman spreading section or power of molecules Raman signal can be considerably enhanced by the effect of surface-enhanced Raman scattering (SERS).[1] According to Canamares et al.,2 the SERS process sourcing from the chemical and electromagnetic enhancements involving molecules and metal structures in close propinquity was foremost seen in 1974 by Fleischmann et al. What’s more, the standard SERS effect can increase in intensity if Raman over innervations vicinity can attain roughly eight magnitude orders, whereas an augmentation of 14–15 magnitude orders has as well been exhibited in a restricted area for just one molecule. Surface-enhanced phenomenon discovery has activated development of numerous extremely sensitive detection technologies, especially in biomedical engineering, analytical chemistry, and life sciences, like surface-enhanced fluorescence, surface-enhanced second-harmonic production, and production of the surface-enhanced sum frequency. 3 SERS is perceived first and foremost for analytes taken up onto metal surfaces of alkali (K, Na, Li) or coinage (Cu, Ag, Au), with the wavelength of innervations/excitation close to or in the detectable area.4

[2] Hypothetically, every metal can exhibit SE, but the alkali and coinage metals gratify quantifiable requisites and offer the strongest improvement. Brolo and Addison 4 posit that metals like Pt or Pd display improvements of roughly 102-103 close to ultraviolet for excitation. SERS significance is that the surface sensitivity and selectivity lengthens RS efficacy to an extensive range of interfacial machines, which beforehand were unavailable to RS for the reason that RS was not sensitive at the surface. These comprise ambient and in-situ examination of organic, catalytic, biological and electrochemical systems. Canamares et al.,2 talk about the optional surface methods whose constraints consist of a desire for stumpy wave number range, ultra high vacuum (UHV) requisites, bulk phase imposition, and diminutive sensitivity. In essence, SERS can be conducted within states that are ambient and have a wider range of wave number, is moderately susceptible and is surface discerning.

Fig 1: Designing nanostructures with optimized surface-enhanced Raman scattering

[3]A further normally fascinating feature of SERS is accredited to its spatial decree. making use of special metallic nanostructures local optical fields, SERS can offer resolutions that are lateral enhanced as compared to 10 nm, which is arguably two magnitude orders under the limit of diffraction and even lesser in contrast with the common near field microscope tips resolution. Because of its premature days, the effect of SERS has as well been predominantly interesting in the domain biochemistry and of biophysics for numerous reasons.3 The most exhilarating for studies of biophysical may possibly be the SERS effect trace analytical abilities mutually with its sky-scraping structural selectivity as well as the chance to determine Raman spectra from tremendously diminutive volumes. Mainly, capabilities of single-molecule expose exhilarating contexts for SERS as an instrument in medication laboratory as well as for fundamental biophysics research, where SERS is able to present fascinating novel features contrasted with fluorescence, which is extensively utilised as a biophysics tool for single-molecule spectroscopy. 5

According to Han et al., 5 one of the most stunning usages of single-molecule SERS may perhaps come into view in the domain of hasty DNA sequencing through Raman spectroscopic classification of definite fragments of DNA down to recognition of single bases that are structurally sensitive exclusive of applying radioactive or fluorescent or labels. Fascinating features in biophysical SERS uses come as well from making use of SERRS. Besides, the augmented resonance Raman scattering (RRS), cross sections, has the benefit of upper specificity, given that the resonance Raman continuum is subjugated by vibrations of molecules that are connected to the molecule part accountable for the suitable transition of resonant electronic.4 What’s more, SERRS unlocks fascinating chances for enormous biomolecules, whereby it permits chromophoric selective vibrational probe systems like carotenoids, pheophytin and chlorophyll. As an extra benefit, the fluorescence setting, which has the ability to make spectroscopy of RRS exceedingly complicated, has been slaked in numerous SERR trials by novel channels known as nonradiative decay channels offered by the SERS-dynamic metal.

[4]2.0 Fundamental Principles of SERS

2.1 Electromagnetic enhancement

Better element of the entire SERS enhancement is owing to an electromagnetic enhancement method that is an ultimate result of the metal roughness features existence at the surface of the metal.6 According to Jiang et al., 6 these attributes can be built up in various means; for instance: oxidation-diminution cycles on surfaces of electrode; metal spheroid sets generated through lithography; metal particles vapour deposition onto the substrate; and deposition of metal over a polystyrene nanospheres deposition mask. Each of the above listed means leave the surface of metal with diminutive metal collections of particles that can serve as attributes of metal roughness. Guillot and de la Chapelle7 posit that EM enhancements are in fact, and ought to be, non-selective in chemical form; i.e., offering the equivalent enhancement for diverse analyte particles adsorbed on the similar sort of metal surface.

[5]Conversely, CO and N2 enhancements vary by a factor of 200 and this presents proof for methods excluding the EME, as well as the chemical enhancement is mixed up given that the two molecules polarizabilities are roughly the equivalent, and adsorption orientation differences could not report for a surface enhancement disparity of such scale. Arguably, chemical enhancement study is complicated as compared to EME for the reason that most examined surfaces are coarsened and the CE and EME effects cannot be unconnected.8 Nevertheless, Futamata and Maruyama8 examined pyromellitic dianhydride (PMDA) taken up onto an atomically flat copper surface,. Whereby he noted that the electromagnetic surface donations are diminutive and well comprehended, making it simpler to examine the chemical enhancement method. Besides, Sun, et al.9 noted a 30x enhancement, and illustrated it as a Resonance Raman type of occurrence and other methods except the CE were cautiously discarded. Given that, crystal or molecule Raman intensity is relative to the electromagnetic field of the incident, each crystal or molecule the enhancement factor (EF) explained by Equation below.

2.2 Chemical Enhancement [6]

According to Otto,10 the chemical enhancement (CE) method offers two of magnitude enhancement or an order to the intensity of Raman signal. In essence, it is comprehended in a smaller extent as compared to the electromagnetic enhancement, but conveys a number of exciting contemplations to a methodical argument of SERS. Fundamentally, the molecules or a particle adsorbed on the surface unavoidably contact with the surface.11 Chemical enhancement subsists as a consequence of this contact, illustrated in a number manner. Lee et al.12 believe that the proximity of metaladsorbate may possibly permit electronic coupling pathways from which new charge-transfer intermediaries surface that contain advanced cross-sections of Raman scattering than how the analyte does when not taken up on the surface and this is extremely much akin to a Resonance Raman effect. An additional description is that the adsorbate molecular orbitals widened into the metal conducting electrons, changes the chemistry of analyte. It is fascinating to note that the chemical enhancement effect may perhaps be a modification in the dissemination section; the analyte chemical environment altering because of its contact with the metal, while the electromagnetic enhancement effect was an alteration in the analyte molecules concentration that did disperse, not an alteration in dissemination cross-section. 10

[7]Based on Palonpon et al.11 study, chemical enhancement is an additional method that is accountable for the SERS effect, wherein the molecule Raman polarisability taken up onto a surface of the metal is improved. For example, in spite of N2 and CO having analogous Raman cross sections, the intensity of Raman for the two distinct particles can vary by a factor of roughly 200. The above figure demonstrates the charge transfer (CT) paradigm to elucidate the above examination, demonstrating the metal electron transfer with atomic scale roughness (ASR) taken up with analytes. Lee et al.12 are of the view that when a photon intrudes on a metal, the metal electron is energized and shifts into adsorbed molecule electrons affinity levels onto the metal. For this reason, molecule relaxation pursues a distinct novel stability, different the usual course of the equivalent molecule devoid of any metallic interaction. Lee et al.12 affirm that the molecule may possibly reside in the novel energized vibrational condition, even following the energized electron moving back to the metal and joins with a metal-hole, generating a Raman-scattered photon.  For that reason, the SERS effects disparity amid N2 and CO can be demonstrated by the unrelated N2 and CO adsorption levels on the silver substrate. Given that CO can comparatively be well taken up on the silver substrate as compared to N2, then the charge transfer can occur flanked by the absorbate and metal and as a result, chemical enhancement is achieved and steers SERS enhancement.12

[8]2.3 localised surface Plasmons (LSP)

As an effectual method to prevail over the classical optics diffraction limit as a result of the momentum and energy disparity between photons and electrons, localized surface plasmons (LSPs) which refers to the set of electronic oscillations in the metallic nanostructures excited or energized by the outer radiation have incarcerated a great deal of research interest.13 According to Bian et al.,14 the electromagnetic field incarceration at the sub-wavelength volume plus this field Purcell enhancement on the order of the LSP resonance quality factor (Q) is realized by the implementation of these polariton methods. Bian et al.14 further note that this Purcell enhancement effect is comparable to the microcavity Purcell effect in the midst of a quality factor Q, where the combination amid a microcavity form and an exciton is approved by the field internment in an ultrasmall volume V. Arguably, this outcome can improve the photon conditions density, which is relative to the impulsive discharge decay rate, steering the luminescence Purcell factor (Fp) enhancement.15 Apparently, this Fp has the ability to indirectly distinguish the efficiency of coupling, which is affected in particular by the metallic nanostructures species and sizes, the distance (d) connecting luminescence matrix excitons and LSPs, and the emitted photons energy (hνD = hc/λD). What’s more, these parameters effects on Fp and the estimation of the excitons standard position in the luminescence matrix are predominantly significant for the supplementary luminescence properties optimization of the dynamic matrix.13

Fig 4: Sensing using localised surface Plasmon resonance sensors

[9]2.4 localized surface Plasmon resonance

The noble metal nanoparticles size-reliant optical properties have been widely examined in the past two decades as well as taken advantage in numerous applications and remarkably for chemical sensors, such as bioor.16 According to Shin et al.,16 the core factor of such properties is the Localized Surface Plasmon Resonance (LSPR) recognized to produce a well-built local electromagnetic field enhancement at the metallic nanoparticles surrounding area. This incident is the foundation of the SERS, whereby the Raman intensity, inherently feeble, of a molecule taken up at a surface of metallic nanoparticles gains from this well-built enhancement of local field to offer the supposed electromagnetic effect. Arguably, SERS effect can be attained with a range of metallic nanostructures types; For instance, Huang, et al.17 worked on SERS effect attained by gold colloidal solution deposition. However, this chemical method of SERS substrate combination offers two key shortcomings; foremost, the related LSPR is not simply adjusted to the excitation wavelength, and subsequently, SERS and nanoparticle structures and reaction are not reproducible. In contrast, nanolithographic mechanisms are recognized to offer an extremely superior control of the nanoparticles shape and the size placed onto the substrate.18[10]

In essence, this permits an accurate management of the LSPR and, as a result, of the Raman enhancement. Fahnestock et al.18 posit that in the case of nanotriangle or nanocylinder arrays, the finest Raman enhancement is attained for a Localized Surface Plasmon Resonance location amid the Raman wavelengths and excitation while it is not at all times the case for shapes such as nanowires. Additionally, in this last instance, utilising an enormous array of cylinder diameters, Huang et al.17 premeditated the nanocylinder arrays SERS efficiency at two excitation wavelengths namely: 785 and 632.8nm, to establish the effect of the LSPR order and position onto the intensity of SERS. [11]

Fig 5: Localized Surface Plasmon Resonance

 

The function of LSPR produced by the substrate of nanolithographied has undoubtedly been recognized. Undeniably, as it is renowned, LSPR position is red-moved for superior mode of resonance and diameter, and in Shin et al.16 study enormous nanocylinders was observed with diameter more than 300 nm.

Fig 6: LSPR Position vs. Cylinder diameter evolution for the first order

[12]2.5 Hotspots

The most examined effect of the SERS hotspots is the analytes therein large Raman enhancement, though it is an unpopular effect, the development of hotspots may possibly cause the ensnared analytes to alter their molecular orientation, which sequentially steers enunciated alterations in SERS fingerprints.19 In this regard, Asiala and Schultz19 laid bare this effect by generating and distinguishing colloidal solutions hotspots. In essence, anisotropically useable Au nanorods were produced, wherein the surfaces were purposely summed up by acrylic acid [13](polystyrene-block-poly), leaving behind the ends un-summarized and useable by a 4- mercaptobenzoic acid of SERS analyte.20 Immediately after salt treatment, Zenidaka et al.21  claim that these nanorods accumulate into linear chains, generating hotspots that integrate the analyte of SERS. Large SERS enhancement was noticed in Zenidaka et al.21 study, especially for a number of feeble/motionless SERS modes that were unavailable in the novel spectrum prior to the development of hotspots. Comprehensive spectral examination revealed that the SERS fingerprint variations were reliable with the analyte molecules reorientation from almost standing to tilted/parallel conformation onto the surface of AU21

[14]Fig 7: Capillarity-constructed reversible hot spots for reversible molecular trapping

Basically, SERS fingerprints offer extremely explicit classification of the individual analytes integrated with the enormous improvement at hotspots, this might steer influential basis for multiplexed detection and sensing.21 For this reason, it is vital to investigate the SERS fingerprints variability and particularly the exclusive control wielded by a hotspot. Furthermore, the SERS spectrum peaks source from a variety of analyte vibrational modes; thus, it is usually acknowledged from hypothetical viewpoints that the molecular point of reference on the surface of metal would have an effect on both the chemical enhancement. This arguably is responsive to the metal analyte multifarious orbital couplings, and electromagnetic enhancement, which according to Wen et al.20 is the toughest at the time when a vibrational mode lines up with the electric field locally. Whilst it is plausible that hotspots may possibly put forth physical stress on the analytes that are rapped, leading to changes in molecular orientation, hardly any reports recorded the SERS response variations as a product of hotspot development.

Definitely, just a small number of reports reviewed the reliance of SERS reaction on other investigational states like the analyte concentration at the time of its adsorption, which was over and over again comprehended as disturbing the density of the molecular and consequently the orientation. Asiala and Schultz19 assert that the setbacks in reviewing SERS variation that is hotspot-induced are intrinsic in the classification techniques, which are up to now predominantly anchored in single-nanocluster or single-molecule methods. What’s more, the analytes photodamage and especially the single-molecule blinking SERS, supplement to the vagueness in allocating the spectral alterations source. Conspicuously, the insufficient ability in making consistent and reliable SERS hotspots has limited the development of both ensemble and single-molecule studies. Hitherto, for the most part of the studies produced hotspots by chemical connecting of metal nanoparticles or salt-induced aggregation which unavoidably steered combination of nanoclusters of diverse structures and sizes.19

[15]5.0 Conclusion

Conclusively, Raman spectroscopy is arguably a vibrational spectroscopic method that determines processes of inelastic light-scattering. It can offer explicit materials for spectroscopic fingerprints in molecular make-ups and works of art. Raman dissemination amplification in SERS effect is usually produced through two methods: (1) chemical enhancement attributable to the rise of Raman cross-section at the time the lattice or molecule is makes contact with metal nanostructures, and (2) electromagnetic-field enhancement through optical fields’ localization in metallic nanostructures. Ultimately, the study has provided a critical analysis of various SERS fundamental principles such as Hotspots, localized surface Plasmon resonance, and localised surface Plasmons.

 

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