Review Article | Open Access
Jiangtao Lv, Eunice Sok Ping Leong, Xiaoxiao Jiang, Shanshan Kou, Haitao Dai, Jiao Lin, Yan Jun Liu, Guangyuan Si, "Plasmon-Enhanced Sensing: Current Status and Prospects", Journal of Nanomaterials, vol. 2015, Article ID 474730, 10 pages, 2015. https://doi.org/10.1155/2015/474730
Plasmon-Enhanced Sensing: Current Status and Prospects
By combining different plasmonic nanostructures with conventional sensing configurations, chemical/biosensors with significantly enhanced device performance can be achieved. The fast development of plasmon-assisted devices benefits from the advance of nanofabrication technology. In this review, we first briefly show the experimental configurations for testing plasmon enhanced sensing signals and then summarize the classic nanogeometries which are extensively used in sensing applications. By design, dramatic increment of optical signals can be obtained and further applied to gas, refractive index and liquid sensing.
The rapid development of plasmonics [1–3] and plasmon-related devices [4–12] paves the way for controlling electromagnetic waves at the nanoscale. The coherent free electron excitations (i.e., surface plasmon resonances, SPRs) which exist at the metal/dielectric interfaces are normally generated by illuminating light to metallic or metallic-dielectric hybrid structures. As yet, various plasmon-assisted optical components have been conceived and experimentally demonstrated, including waveguides [13–19], photon sorters [20–22], absorbers [23–25], color filters [26–28], and switches [29, 30]. SPR based optical sensors [31–56] are another important research field since they have found numerous useful applications in detecting and characterizing chemical and biological molecules.
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.
2. Configurations of SPR-Assisted Sensing
Figure 1(a) illustrates the classic Kretschmann configuration  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 , 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.
3. 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 [59–62], electron-beam lithography (EBL) [63–67], nanoimprint [68–71], and interference lithography [72–75]. Various plasmonic designs have been demonstrated to enhance the sensing effect. Figure 2 shows typical structures including 1D nanogratings (Figure 2(a)), 2D nanodots (Figure 2(b)), nanoholes (Figure 2(c)), and nanomushrooms (Figure 2(d)). FIB milling was applied to fabricate the nanogratings illustrated in Figure 2(a) . 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  and nanoholes  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)  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 . 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  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 . 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 . 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  by covering a thin protein layer on the sample surface.
Quantitative detection of biomolecules with a wide range of concentrations (from 3.9 μg/mL to 1000 μg/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  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 cross-shaped nanocavity array  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 . 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 . For clarity, dips are normalized and replotted in Figure 6(b). 1015 nm/RIU and 80–108 figures of merit are determined.
4. 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 [89–91] are emerging devices which have great potential and important applications. A graphene-MoS2 hybrid sensor  and an incident-angle tunable graphene-plasmonic sensor  are demonstrated most recently. The proposed graphene-MoS2 hybrid sensor has a huge phase-sensitivity enhancement compared with the SPR sensing scheme with only graphene coating since MoS2 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.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partially supported by the National Natural Science Foundation of China (Grant no. 61405031). Leong and Liu acknowledge the Joint Council Office (JCO) of the Agency for Science, Technology and Research (A*STAR) under Grant no. 12302FG012. Lv also acknowledges Natural Science Foundation of Hebei Province (Grant no. F2014501127), Science and Technology Research Funds for Higher Education of Hebei Province (Grant no. ZD2015209), and Fundamental Research Funds for the Central Universities (Grant no. N142304006).
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