Plasmon-Enhanced Sensing : Current Status and Prospects

1College of Information Science and Engineering, Northeastern University, Shenyang 110004, China 2Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602 3Australian Research Council Centre of Excellence (ARC CoE) for Advanced Molecular Imaging, La Trobe University, Melbourne, VIC 3086, Australia 4Department of Chemistry and Physics, La Trobe Institute for Molecular Sciences (LIMS), La Trobe University, Melbourne, VIC 3086, Australia 5Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China 6School of Electrical and Computer Engineering, RMIT University, Melbourne, VIC 3001, Australia 7School of Physics, The University of Melbourne, Melbourne, VIC 3010, Australia

It is well known that surface plasmons can propagate along the metal-dielectric interface from tens to hundreds of microns and decay evanescently in the vertical direction.Based on the fact that SPR is sensitive and highly dependent on the dielectric environment, the shift of resonance in optical spectrum can be used to quantify the change of surrounding medium since analyte can interact with electromagnetic waves which are tightly confined on the structure surface.Note that nonpropagating plasmons are also useful for sensing applications due to greatly enhanced field intensity at the resonance (localized surface plasmon resonance, LSPR) due to collective oscillations.Here, we first briefly retrospect the classical experimental configurations for plasmon-enhanced sensing and then summarize typical designs that can remarkably strengthen the optical signal.The sensing effect will be reviewed both theoretically and experimentally.

Configurations of SPR-Assisted Sensing
Figure 1(a) illustrates the classic Kretschmann configuration [57] which is most frequently applied to excite SPR.A prism is normally used to match the wave vectors between surface plasmons and the incident light.Alternatively, patterned structures (e.g., gratings) can also be employed to generate surface plasmons, as shown in Figure 1(b).Additional wave vectors are offered by patterned structures which further help convert the incident light to surface plasmon waves.
To take advantage of the unique properties of SPR, one needs to combine the analyte with experimental setup which can excite surface plasmons.This can be realized by covering the metal surface with a thin layer of molecules [58], as illustrated in Figure 1(c).More complicatedly, complex designs of nanostructures with well-aligned arrays can be utilized to further generate optical response at different frequency bands.Analyte tightly adhered to patterns (e.g., nanotrenches) as shown in Figure 1(d) interacts with excited SPR and produces detectable resonance shift in the spectrum.

Plasmonic Nanostructures for Enhanced Sensing Applications
A variety of patterning methods have been developed to fabricate nanostructures which can be further used for enhancing the serviceable signals.Typically, two different processes are categorized: top-down and bottom-up fabrication techniques.The former includes focused ion beam (FIB) milling [59][60][61][62], electron-beam lithography (EBL) [63][64][65][66][67], nanoimprint [68][69][70][71], and interference lithography [72][73][74][75].Various plasmonic designs have been demonstrated to enhance the sensing effect.FIB milling was applied to fabricate the nanogratings illustrated in Figure 2(a) [76].One can see that the sidewalls are not completely straight, which is caused by material redeposition during milling.Although the redeposition effect is almost inevitable in all ion-involved milling processes, it is still possible to minimize the effect and obtain a smooth surface under optimized conditions.Large area 2D nanorods [77] and nanoholes [78] shown in Figures 2(b) and 2(c) were fabricated by interference lithography.Note that either lift-off or etching is needed to transfer patterns from resists to target materials using interference lithography to define patterns, which is different from direct FIB drilling.It is also worth mentioning that the cross sections of the fabricated structures may not present vertical sidewalls, as illustrated in the inset of Figure 2(b).Using interference lithography followed by thermal evaporation, nanomushrooms (gold caps on photoresist pillars) [79] shown in Figure 2(d) were obtained on a quartz substrate.One can also observe fabrication imperfections from the SEM image.Except for the regular structures (well-aligned arrays) discussed above, various designs with irregular shapes have also been reported.As shown in Figures 3(a) and 3(b), gold nanoislands [80,81] were constructed by evaporation followed by annealing, which can be used for chemical/ biosensing in the transmission localized surface plasmon resonance (LSPR) mode.The structure in Figure 3(c) consists of a cross and a bar [82].Such a hybrid using an "X" and "I" shaped particles can generate Fano resonance caused by two relevant modes which are known as bonding mode (superradiant mode) and antibonding mode (subradiant mode).This nanoresonator can be applied in sensing applications since coherent coupling of bright and dark plasmon modes in this hybrid system is expected to produce Fano interference with high quality factors.Using nanoporous anodic alumina oxide (AAO) as templates, bimetallic (gold core with palladium shell) nanorod metamaterials with high aspect ratios [83] were fabricated using a self-organization technique (Figure 3(d)), which can find important applications in hydrogen gas sensing.
The key factor for strengthening the optical signals is the electric field enhancement assisted by plasmonic nanostructures.This has been extensively verified both theoretically and experimentally.As illustrated in Figure 4, near field increment generated by plasmonic nanostructures is obvious.By using different designs (e.g., nanocube particles and nanohole cavities) significantly increased near field intensity can be achieved.Enhanced electromagnetic fields are verified using finite difference time domain (FDTD) calculations.In Figures 4(a) and 4(b), one can see that different field intensity distributions are observed for different resonance modes (peaks) in scattering spectrum when a cubic silver particle is in contact with a dielectric substrate [84].The silver cube with 90 nm length of side is supported by a glass substrate.Since the dependence of field distribution can be either on the top of the particle (Figure 4(a)) or on the substrate (Figure 4(b)), one can apply this dependency to enhance the detecting signals for different kinds of analytes.The field distribution in Figure 4(c) demonstrates the enhancement effect using classic nanohole structures [85].Exosomes supported by the nanohole plasmonic structures are thus sensed with larger signals.Since the typical size of exosomes is from 50 nm to 100 nm in diameter (the diameter and period of the hole array are 200 nm and 450 nm, resp.), increased electromagnetic fields are confined in this range.Still using the classic nanohole arrays (200 nm diameter and 600 nm periodicity), Cetin et al. demonstrated a plasmonic on-chip sensing platform [86] by covering a thin protein layer on the sample surface.
Quantitative detection of biomolecules with a wide range of concentrations (from 3.9 g/mL to 1000 g/mL) is thus realized as shown in Figure 5(a).Spectral response (normalized transmission) reveals that the resonance peak redshifts with increasing concentrations.Liu and coworkers proposed a perfect absorber [87] using a thin layer of plasmonic nanodisks and further demonstrated its sensing application.Blueshift of resonance dips in the reflectance spectrum as a function of frequency was realized, as shown in Figure 5(b).More importantly, multispectral sensing can also be achieved by using proper plasmonic designs.A hexagonal crossshaped nanocavity array [88] was proposed and theoretically demonstrated to achieve narrowband near-unity absorption for sensing applications, as shown in Figure 5(c).We can see that the shift of resonance peaks in absorption spectrum is obvious for very small refractive index variation (1.000 to 1.040 with only 0.005 increments) from Figure 5(c), which is potentially useful for detecting flammable gases and even poisonous materials [88].Sensitivities of 448, 504, 538, and 564 nm/RIU (from left to right) for the four resonance peaks are obtained, respectively.
Moreover, much higher sensitivity and figure of merit (near to the theoretical limit) have also been experimentally demonstrated using the nanomushrooms shown in Figure 2(d).As shown in Figure 6(a), the resonance dips in reflectance show distinct redshift with larger refractive indices [79].For clarity, dips are normalized and replotted in Figure 6

Conclusions and Outlook
In summary, we have reviewed the typical sensing platforms using plasmonic designs with various nanostructures.By covering analytes on a sample surface, functional sensors with high performance (refractive index sensitivity and figure of merit) can be readily obtained since the electromagnetic field is dramatically enhanced at plasmon resonances.Different geometries (both well-aligned arrays and patterns with arbitrary shapes) can be utilized to actualize the enhanced sensing effect.Ultrahigh sensitivities enable various applications in a wide range of research domains.Since higher sensitivity and faster respond speed are always desired, future plasmonic sensors need ultrasensitive and ultrahigh speed performance.Moreover, sensing devices with high throughput and scalable detection are essential by combining plasmonics with microfluidics.
Graphene-related sensors [89][90][91] are emerging devices which have great potential and important applications.A graphene-MoS 2 hybrid sensor [89] and an incident-angle tunable graphene-plasmonic sensor [90]   has high absorption efficiency.For the incident-angle tunable graphene-plasmonic sensor, a TM-polarized light is needed to illuminate the structure beyond the critical angle with the help of a prism.Graphene has unsubstituted advantages such as high electronic mobility, large specific surface area, and preponderance of exposed edge planes to greatly increase charge storage and universal optical conductivity from visible to infrared frequencies and it has found tremendous application and great potential in composing bio/chemosensors.

Figure 1 :
Figure 1: (a) Schematic showing the Kretschmann configuration for surface plasmon excitation.(b) Grating can also provide additional wave vector components and therefore assist the conversion from incident light into surface plasmon waves.(c) Schematic diagram showing Kretschmann configuration conventionally employed for coupling incident radiation to surface plasmons with a thin layer of molecules on the metal surface.(d) Narrow groove plasmonic grating structures illustrating the important dimensions and parameters.The incident and reflected radiation are indicated by symbols "" and "," respectively.(a)-(b) and (c)-(d) are adapted from [57, 58], respectively.(a)-(b): Copyright 2014, Multidisciplinary Digital Publishing Institute.(c)-(d): Copyright 2011, Optical Society of America.

Figure 2 :
Figure 2: SEM images showing four different plasmonic nanodesigns.(a) SEM image of 1D nanogratings patterned on a silver film with 420 nm period and 110 nm slit-width.The inset illustrates the schematic cross section of the nanogratings on a glass substrate.(b) Top-view SEM image of a nanodisk array with 500 nm periodicity.The inset is the oblique view of the disks.(c) Top-view SEM image of a nanohole array.(d) Oblique view of nanomushrooms fabricated by interference lithography and thermal evaporation.Scale bars in (b) and (d) represent 300 nm and 200 nm, respectively.(a), (b), (c), (d) are successively adapted from [76-79].(a): Copyright 2014, American Institute of Physics.(b): Copyright 2014, Elsevier.(c): Copyright 2014, American Institute of Physics.(d): Copyright 2013, Nature Publishing Group.

Figure 4 :
Figure 4: (a) and (b) The field intensities at different resonance peaks of a single silver nanocube in contact with a glass substrate.(c) The near field distribution showing enhanced electromagnetic fields tightly confined near a periodic nanohole surface.One can see that the field distribution overlaps with the size of exosomes captured onto the sensing surface.(a)-(b) and (c) are adapted from [84, 85], respectively.(a)-(b): Copyright 2005, American Chemical Society.(c): Copyright 2014, Nature Publishing Group.

Figure 5 :
Figure 5: (a) Spectral response of the plasmonic sensors functionalized with different protein IgG concentrations ranging from 3.9 g/mL to 1000 g/mL.(b) Reflectance spectra of an absorber sensor designed for water as reference medium.(c) Absorption spectra of the plasmonic nanostructure under a low refractive index environment.(a), (b), (c) are adapted from [86-88], respectively.(a): Copyright 2014, Nature Publishing Group.(b): Copyright 2010, American Chemical Society.(c): Copyright 2014, American Institute of Physics.

Figure 6 :
Figure 6: (a) Reflectance spectra of the gold mushroom array immersed in glycerine-water mixture solutions with varying compositions at the incidence angle of 33.3 ∘ .(b) Normalized reflectance for D1 in the spectral region indicated with the dashed box.(a) and (b) are adapted from [79].(a)-(b): Copyright 2013, Nature Publishing Group.