Surface-Enhanced Raman Sensing of Semi-Volatile Organic Compounds by Plasmonic Nanostructures

Facile detection of indoor semi-volatile organic compounds (SVOCs) is a critical issue to raise an increasing concern to current researchers, since their emissions have impacted the health of humans, who spend much of their time indoors after the recent incessant COVID-19 pandemic outbreaks. Plasmonic nanomaterial platforms can utilize an electromagnetic field to induce significant Raman signal enhancements of vibrational spectra of pollutant molecules from localized hotspots. Surface-enhanced Raman scattering (SERS) sensing based on functional plasmonic nanostructures has currently emerged as a powerful analytical technique, which is widely adopted for the ultra-sensitive detection of SVOC molecules, including phthalates and polycyclic aromatic hydrocarbons (PAHs) from household chemicals in indoor environments. This concise topical review gives updated recent developments and trends in optical sensors of surface plasmon resonance (SPR) and SERS for effective sensing of SVOCs by functionalization of noble metal nanostructures. Specific features of plasmonic nanomaterials utilized in sensors are evaluated comparatively, including their various sizes and shapes. Novel aptasensors-assisted SERS technology and its potential application are also introduced for selective sensing. The current challenges and perspectives on SERS-based optical sensors using plasmonic nanomaterial platforms and aptasensors are discussed for applying indoor SVOC detection.


Introduction
Functional plasmonic nanostructures have gained increasing interest in energy and environmental fields [1]. SERS has recently been introduced for effective sensing combined with noble metal systems by introducing hotspots to enhance electromagnetic enhancements [2,3]. Biosensing [4,5] can also be achieved using novel nanostructures [6]. Semivolatile organic compounds (SVOCs) have a range of boiling points, from 260 to 400 • C as a sub-group of volatile organic compounds (VOCs) [7]. SVOCs, including plasticizers, flame retardants, per-fluorinated compounds, antioxidants, per-and polyfluoroalkyl substances, etc., have been widely used as additives in several commercial products such as textiles, polymers, cleaning products, electronic devices, plastic items, etc., which are usually used in houses [8]. SVOCs can slowly and directly be emitted from source materials to the indoor environment under various phases, such as dust fractions [9], and gas [10,11]. Subsequently, these released SVOCs can critically impact health due to their uptake via various pathways such as human skin, clothing, and hair [12][13][14]. Polybrominated diphenyl ethers [15] have been found to enhance emissions from indoor sources by contacting dust directly, causing potential health risks, whereas the transport of phthalates in indoor air

Pure Noble Metal Nanostructured Material-Based SERS Sensor
The most significant factor of the successful SERS sensor is the type of plasmonic nanostructured materials that have been distributed in SERS-active platforms. Among many materials, noble metals, such as Au and Ag, have emerged as potential materials that have contributed to the fabrication of several novel SERS-active platforms [25,32]. Furthermore, the sensitivity of the SERS method depends on the size and geometrical nanostructures of noble metals due to the plasmonic resonance phenomena occurring on their surface. Therefore, a series of designed strategies for pure noble metal nanostructured materials have been discovered that aim at maximizing enhancement factors, not only enhancement of electromagnetic fields but also amplification of SERS signals. For example, many studies have been recently investigated, including silver nanoparticle (AgNP)-based SERS trace detection of 16 typical polycyclic aromatic hydrocarbons [61], AuNP-based [62] and AgNP-assisted [63,64] SERS rapid determination of bisphenol A (BPA), AuNP arrays-introduced SERS sensitive detection of butyl benzyl phthalate (BBP) [65,66], and silver nanorod-based SERS detection of benzo(a)pyrene (BaP) [67].
As shown in Figure 2, a series of studies on silver nanorod-based, AgNP-based, and AuNP-based SERS methods have been published on trace detection of various indoor SVOCs as well as BaP, BBP, and polycyclic aromatic hydrocarbons. Figure 2A introduces an efficient SERS detection of BBP using ~18 nm spherical AuNPs; first, BBPs have been isolated by extracting with organic solvent as cyclohexane (CYH). Subsequently, an

Pure Noble Metal Nanostructured Material-Based SERS Sensor
The most significant factor of the successful SERS sensor is the type of plasmonic nanostructured materials that have been distributed in SERS-active platforms. Among many materials, noble metals, such as Au and Ag, have emerged as potential materials that have contributed to the fabrication of several novel SERS-active platforms [25,32]. Furthermore, the sensitivity of the SERS method depends on the size and geometrical nanostructures of noble metals due to the plasmonic resonance phenomena occurring on their surface. Therefore, a series of designed strategies for pure noble metal nanostructured materials have been discovered that aim at maximizing enhancement factors, not only enhancement of electromagnetic fields but also amplification of SERS signals. For example, many studies have been recently investigated, including silver nanoparticle (AgNP)-based SERS trace detection of 16 typical polycyclic aromatic hydrocarbons [61], AuNP-based [62] and AgNP-assisted [63,64] SERS rapid determination of bisphenol A (BPA), AuNP arraysintroduced SERS sensitive detection of butyl benzyl phthalate (BBP) [65,66], and silver nanorod-based SERS detection of benzo(a)pyrene (BaP) [67].
As shown in Figure 2, a series of studies on silver nanorod-based, AgNP-based, and AuNP-based SERS methods have been published on trace detection of various indoor SVOCs as well as BaP, BBP, and polycyclic aromatic hydrocarbons. Figure 2A introduces an efficient SERS detection of BBP using~18 nm spherical AuNPs; first, BBPs have been isolated by extracting with organic solvent as cyclohexane (CYH). Subsequently, an amount Nanomaterials 2021, 11, 2619 4 of 20 of ethanol has been added to this mixture, inducing the self-assembly of AuNPs at the interface between CYH and water. Within 10-20 s, aggregated AuNP arrays have been formed with many hotspots. After being transferred onto the surface of clean silicon wafers, self-assembled AuNP arrays containing BBPs have been checked by means of SERS. Figure 2B shows a schematic diagram of an efficient colorimetric and SERS sensing of BBP d using β-cyclodextrin (β-CD)-stabilized AuNPs. β-CD molecules have played significant roles of Raman probes to capture BBP. Due to the AuNP-correlated hotspots, BBP could be quantified either with the naked eye or by means of SERS. Figure 2C demonstrates a silver nanorod-based SERS method for a trace detection of BaP in real samples as well as river water and soil. Due to the unique structures of SERS substrates with highly dense hotspots, the detection limit of BaP has been estimated to be as low as 1 ppm and 10 ppm in river water and soil, respectively. Figure 2D illustrates a schematic diagram for AgNP-assisted SERS detection of 16 typical PAHs in water. Based on plasmonic AgNPs and the liquid extraction step, the SERS method has been successfully applied to a trace detection of PAHs in real water environments containing contaminants. Herein, PAHs have been isolated from water by extraction with a nonpolar organic solvent. Subsequently, three steps including separation, transfer, and volatilization have been performed. Finally, acetonitrile has been used to elute the extracted PAHs to obtain solution for measured SERS spectra.
An efficient SERS substrate has been developed by the vertical array of Ag nanoplates on the substrate's surface [47]. Since Ag nanoplates have been well-aligned on surface platforms, inducing not only achieving gaps between near Ag nanoplates but also obtaining many sufficient hotspots on the whole substrate. This Ag nanoplate-assembled substrate exhibits strong effective plasmon resonance phenomena, which have been applied for SERS sensitive detection of 3,3 ,4,4 -tetrachlorobiphenyl (PCB-77). Furthermore, in mixed solutions, this SERS substrate has successfully distinguished the characterization of Raman peaks of different PCBs. Thus, Ag nanoplate-assembled substrate based on the vertically well-aligned Ag nanoplate has been a potential SERS substrate for robust, direct, and trace detection of PCBs ( Figure 3).
Moreover, either benzothiazole (BZT) or its derivatives have been used as a popular ingredient in household products due to their pharmaceutical activities and biological features. Therefore, evaluating harmful health effects of their emissions inducing exposure from sources to indoor air is important. Authors have studied the evaporation of BZT from glass surfaces by combining AuNP-based SERS sensor and gas-phase infrared spectrum [44]. BZT exposure early warning is among the major concerns of environmental researchers. This investigation has been reported on optical sensors applications for monitoring BZT directly from glass surfaces. Following the fabrication of a BZT thin film on the surface, the SERS sensors method has been performed by loading AuNPs on glass surfaces. Time-dependent SERS spectral intensities of BZT have decreased, indicating substantial evaporation of BZT from the glass surface to indoor air. Simultaneously, BZT concentration in indoor air has been estimated via infrared spectra.

Hybrid Nanostructured SERS Sensors
Hybrid nanostructured materials have also been widely introduced to fabricate various SERS substrates that are applied in many fields such as biological sensing and environmental monitoring. Hybrid nanomaterials have more advantages of providing the multiple physicochemical properties than pure noble metals. Several studies have been reported for fabricating various novel hybrid nanomaterials exhibiting not only uniform geometries but also unique plasmonic properties, which aimed at developing reproducible SERS platforms. Extensive efforts have been put on the novel and robust SERS-active platforms such as SiO2@Ag-based composite nanospheres [68], plasmonic core-shell Au nanospheres@Ag nanocubes for phthalate plasticizer detection [69], Au@AgNP-assisted highly sensitive detection of BPA [70], up-conversion nanoparticle-decorated AuNPs for the determination of dibutyl phthalate [71], graphene monolayer-coated AgNP-based SERS sensitive detection of BPA [72], and bimetallic plasmonic Au@Ag nanocuboid-introduced SERS detection of phthalate plasticizers [73].
As shown in Figure 4, several studies on hybrid nanomaterial-based SERS methods have been reported on a trace detection of various indoor SVOCs such as PAEs and BPA. Based on the self-oxidative-polymerization of dopamine, the bio-inspired nanostructures of molecularly imprinted polymers (MIP) have been successfully designed as a template for in situ fabrication of AuNPs [74]. The AuNP-coated MIP template has a three-

Hybrid Nanostructured SERS Sensors
Hybrid nanostructured materials have also been widely introduced to fabricate various SERS substrates that are applied in many fields such as biological sensing and environmental monitoring. Hybrid nanomaterials have more advantages of providing the multiple physicochemical properties than pure noble metals. Several studies have been reported for fabricating various novel hybrid nanomaterials exhibiting not only uniform geometries but also unique plasmonic properties, which aimed at developing reproducible SERS platforms. Extensive efforts have been put on the novel and robust SERS-active platforms such as SiO 2 @Ag-based composite nanospheres [68], plasmonic core-shell Au nanospheres@Ag nanocubes for phthalate plasticizer detection [69], Au@AgNP-assisted highly sensitive detection of BPA [70], up-conversion nanoparticle-decorated AuNPs for the determination of dibutyl phthalate [71], graphene monolayer-coated AgNP-based SERS sensitive detection of BPA [72], and bimetallic plasmonic Au@Ag nanocuboid-introduced SERS detection of phthalate plasticizers [73].
As shown in Figure 4, several studies on hybrid nanomaterial-based SERS methods have been reported on a trace detection of various indoor SVOCs such as PAEs and BPA.
Based on the self-oxidative-polymerization of dopamine, the bio-inspired nanostructures of molecularly imprinted polymers (MIP) have been successfully designed as a template for in situ fabrication of AuNPs [74]. The AuNP-coated MIP template has a three-dimensional (3D) nanoplatform, which has been used for selective and sensitive detection of phthalate plasticizers through SERS spectroscopy. These SERS plasmonic nanostructures have been well controlled to produce many "hotspots" on the surface of nanocomposites. Figure 4B illustrates a SERS detection of PAEs in liquid samples based on a plasmonic core-shell nanocuboid of bimetallic Au@Ag. Herein, Au@Ag nanocuboids have been successfully fabricated by a Au nanorod core and a Ag cuboid shell leading to the induction of more effective plasmonic resonance than pristine Au nanorods. Due to a unique structure of Au@Ag nanocomposites that induce strong signals, this SERS sensor indicated that detection limits of BBP and DEHP were estimated to be as low as 10 −9 M. As shown in Figure 4C, a sensitive detection of BPA has been investigated by a solid-phase microextraction (SPME) with a combination of the SERS method. AgNPs were in situ synthesized on the surface of Si fiber leading to form AgNP-coated Si fiber. Subsequently, this structure has been modified with a monolayer of graphene resulting in the generation of a novel nanocomposite including Si fiber, AgNPs, and graphene. This unique structure exhibits not only as a SERS platform but also as an SPME fiber to detect BPA. Due to the effective cooperation of SPME and SERS, this technique shows an excellent capability of BPA detection with a detection limit as low as 1 µg/L. Figure 4D demonstrates a phthalic acid ester (PAE) detection by means of SERS and up-conversion fluorescence. Using AuNPs to decorate up-conversion nanoparticles (UCNPs), a new bimodal platform has been successfully developed in this report. In this nanocomposite, UCNPs have been employed as a fluorescent signal molecule, whereas AuNPs have been used as SERS templates with an aptamer served as a targeted detection of PAEs. This nanocomposites-assisted detection of PAEs has demonstrated the detection limits of 0.0087 and 0.0108 ng/mL for fluorescence sensors and SERS methods, respectively.
An efficient SERS-active substrate has been prepared by the well-designed synthesis of a core-shell zeolite imidazole framework (ZIF-8)-wrapped Au-Ag alloyed nanocrystals in an urchin shape [40]. This novel nanostructure exhibits high-density tips with a thickness of about 100 nm, which has been well-fabricated by adding the pre-formed plasmonic nanoparticles as Au-Ag alloyed nanocrystals into the ZIF-8 precursor. The thickness of the ZIF-8 shell layer has been well-controlled with a size of 20 nm by the hexadecyl trimethyl ammonium bromide (CTAB) concentration. This core-shell configuration of ZIF-8-wrapped urchin-inspired Au-Ag alloyed nanocrystals can be applied as a highly efficient SERS platform for trace detection of hexachlorocyclohexane (HCH) molecules. This work also shows the potential application in detecting small molecules of VOC groups using SERS platforms ( Figure 5). Furthermore, an ultra-sensitive and reproducible SERS platform has been synthesized by the assembly of GO layers and anisotropic nanostructured noble metal hybrid materials, such as Au nanostars (AuNSts) and flower-inspired AgNPs [38]. These well-controlled AgNFs-GO-AuNSts nanostructures have possessed ultra-sensitive detection of PCBs at a level as low as 3.4×10 −6 M due to the coupling effect of multi-dimensional plasmon. This substrate can be used not only to identify Raman peaks of different PCBs in their mixture but also to detect various pollutants in the environment ( Figure 6).  An efficient SERS-active substrate has been prepared by the well-designed synthe of a core-shell zeolite imidazole framework (ZIF-8)-wrapped Au-Ag alloyed nanocryst in an urchin shape [40]. This novel nanostructure exhibits high-density tips with a thi ness of about 100 nm, which has been well-fabricated by adding the pre-formed plasmo nanoparticles as Au-Ag alloyed nanocrystals into the ZIF-8 precursor. The thickness the ZIF-8 shell layer has been well-controlled with a size of 20 nm by the hexadecyl trim thyl ammonium bromide (CTAB) concentration. This core-shell configuration of ZIF wrapped urchin-inspired Au-Ag alloyed nanocrystals can be applied as a highly effici SERS platform for trace detection of hexachlorocyclohexane (HCH) molecules. This wo lapping areas of SERS, fluorescence, colorimetric, and SPR. Extensive efforts have been made toward the potential aptasensor-assisted plasmonic resonance-based efficient sen sors for great advantages in sensor applications. The enhanced signals were attributed to plasmon resonance inducing high sensitivity. In the following part, this review tries to highlight several significant contributions from aptasensor-assisted and plasmonic reso nance-based materials according to selected interesting examples.  Despite extensive efforts to improve plasmonic resonance-based optical sensors, it is continuously needed to conduct additional research to properly design several novel plasmonic nanostructured materials for various platforms using optical sensors, which aims at more sensitive and selective detection of indoor SVOCs. Understanding both plasmonic behaviors and interfaces, the authors have discovered that these new nanomaterial-based aptasensors have demonstrated their potential ability for multi-detection techniques overlapping areas of SERS, fluorescence, colorimetric, and SPR. Extensive efforts have been made toward the potential aptasensor-assisted plasmonic resonance-based efficient sensors for great advantages in sensor applications. The enhanced signals were attributed to plasmon resonance inducing high sensitivity. In the following part, this review tries to highlight several significant contributions from aptasensor-assisted and plasmonic resonance-based materials according to selected interesting examples.

Aptasensor-Introduced Optical Sensors
Recently, aptasensors with short single-stranded oligonucleotides of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) have attracted the attention from many researchers to demonstrate their selectivity and sensitivity features. Because of high affinities, aptasensor-assisted plasmonic resonance optical sensors have been widely adopted as a powerful analytical tool in many fields of applications, such as clinical diagnostics [75], food [76], and environmental pesticide detection [77,78]. The plasmon resonance effects depend not only on the nanostructured materials but also on the configuration of aptasensors. The aptasensor-active optical sensors based on plasmonic resonance have introduced a carbon quantum dot-labeled aptamer-based fluorescent method for dibutyl phthalate [79], an optical fiber-based aptasensor-assisted plasmonic biosensor for BPA [80], asymmetric plasmonic aptasensor-based BPA [81], BPA detection using an AuNPbased colorimetric aptasensor [82], quantum dot aptasensor for di-2-ethylhexyl phthalate (DEHP) [83], localized SPR-assisted electrochemical impedance spectroscopy for DEHP [84], and AuNP-sensitized ZnO nanopencil-based detection of BPA [85].
As shown in Figure 7, several studies on the nanomaterial-assisted aptasensor-based optical sensors methods have been investigated for selective detection of various indoor SVOCs of BPA and DEHP. Herein, the authors have developed several new nanosystems to be applied for various sensor technologies, including colorimetric, fluorescence, and SPR. The colorimetric technique has been known as a normal analytic tool that has been widely adopted for use in detecting analyte molecules via comparing color changes. Fluorescence [86] and SPR [87] technologies have emerged as sensitive and selective analysis that are applied in many fields, such as biomedicine [88], bioimaging [89], and environmental detections [90]. Figure 7A shows an aptasensor-based AuNP-assisted colorimetric detection of BPA. Herein, the aptasensor has been well-designed to consist of AuNPs and a specific 24-bp aptamer of BPA. Aptamer-conjugated AuNP complex has been formed via an electrostatic interaction. This complex will induce an SPR shift in the presence of

Aptasensor-Introduced Optical Sensors
Recently, aptasensors with short single-stranded oligonucleotides of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) have attracted the attention from many researchers to demonstrate their selectivity and sensitivity features. Because of high affinities, aptasensor-assisted plasmonic resonance optical sensors have been widely adopted as a powerful analytical tool in many fields of applications, such as clinical diagnostics [75], food [76], and environmental pesticide detection [77,78]. The plasmon resonance effects depend not only on the nanostructured materials but also on the configuration of aptasensors. The aptasensor-active optical sensors based on plasmonic resonance have introduced a carbon quantum dot-labeled aptamer-based fluorescent method for dibutyl phthalate [79], an optical fiber-based aptasensor-assisted plasmonic biosensor for BPA [80], asymmetric plasmonic aptasensor-based BPA [81], BPA detection using an AuNP-based colorimetric aptasensor [82], quantum dot aptasensor for di-2-ethylhexyl phthalate (DEHP) [83], localized SPR-assisted electrochemical impedance spectroscopy for DEHP [84], and AuNP-sensitized ZnO nanopencil-based detection of BPA [85].
As shown in Figure 7, several studies on the nanomaterial-assisted aptasensor-based optical sensors methods have been investigated for selective detection of various indoor SVOCs of BPA and DEHP. Herein, the authors have developed several new nanosystems to be applied for various sensor technologies, including colorimetric, fluorescence, and SPR. The colorimetric technique has been known as a normal analytic tool that has been widely adopted for use in detecting analyte molecules via comparing color changes. Fluorescence [86] and SPR [87] technologies have emerged as sensitive and selective analysis that are applied in many fields, such as biomedicine [88], bioimaging [89], and environmental detections [90]. Figure 7A shows an aptasensor-based AuNP-assisted colorimetric detection of BPA. Herein, the aptasensor has been well-designed to consist of AuNPs and a specific 24-bp aptamer of BPA. Aptamer-conjugated AuNP complex has been formed via an electrostatic interaction. This complex will induce an SPR shift in the presence of BPA.
Pristine AuNPs exhibit a red color. Because of the presence of BPA leading to the aggregation of AuNPs, the color may change to blue, which can be observed with the naked eye. Based on BPA concentration-correlated blue color changes of aptamer-conjugated AuNP complex, the colorimetric detection has been successfully performed with a limit as low as 0.004 nM. Despite the presence of various BPA analogs, this method achieved a selective detection of BPA. Figure 7B illustrates a magnetic bead-based aptasensor-based quantum dot (QD) and its fluorescence detection of trace DEHP. Herein, in combination with a reduced graphene oxide (rGO)-based screening technique, a novel 60-mer aptamer has been successfully developed using systemic evolution of ligands by exponential enrichment (SELEX) technology. This aptamer-based sensor shows not only good selectivity of DEHP in the presence of other phthalate analogs but also excellent sensitivity as low as 5 pg/mL. As shown in Figure 7B-a, if DEHP is absent, the complex of a specific aptamer-coated magnetic bead conjugated with QD 565 mainly binds to a DNA probe containing QD 655 and its fluorescence is normalized QD 565 . As shown in Figure 7B-b in the presence of DEHP, DNA probes were found to be dissociated from a specific 60-mer aptamer leading to a decrease of normalized QD 655 /QD 565 fluorescence ratios. Figure 7C demonstrates a facile detection of BPA using a combination between a specific aptasensor with SPR of AuNP-activated ZnO nanopencils. Due to many hot electrons from excited AuNPs, Au/ZnO nanocomposites exhibited an increment of photoelectrochemical signals higher than those from initial ZnO nanopencils. Based on a specific aptasensor, this sensor has good selectivity with excellent sensitivity of a detection limit as low as 0.5 nmol/L for BPA in real samples, as well as drinking water and milk.
As shown in Figure 8, a simple detection of trace DEHP has been developed using an aptamer and a Raman reporter of 5,5 -dithio-bis(2-nitrobenzoic acid) (DTNB) molecule [91]. Since AgNP surfaces have been functionalized with DTNB, NaCl-induced aggregations of AgNPs with a silica shell can become an efficient SERS system. These AgNPs-SiO 2 structures have been functionalized with (3-aminopropyl) trimethoxy silane (APTMS) to form a primary amine layer on the surface. Subsequently, the surface of AgNPs-SiO 2 -NH 2 structures has been immobilized with DEHP-COOH molecules by an amide binding through N-ethyl-N -(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and Nhydroxysulfosuccinimide (NHS) as a crosslinking reaction. DEHP aptamers have been immobilized on the surface of magnetic particles. There is a competitive binding between DEHP molecules and SERS silica particles with aptamer-coated magnetic particles. DEHP concentration has been quantitatively determined by comparing the signals of Raman reporter on SERS silica particles. Since DEHP has been widely used in plastic products, aptasensor-assisted SERS sensors can achieve highly selective detection of DEHP, despite interferents with analogous structures, which may provide numerous applications in food and environmental analysis.  [83]. (C) Diagram of the fabricated photoelectrochemical aptasensors using AuNPs and ZnO nanopencils functionalized with BPA aptamers, which was aimed for BPA detection by means of the SPR technique. Adapted from [85].
As shown in Figure 8, a simple detection of trace DEHP has been developed using an aptamer and a Raman reporter of 5,5′-dithio-bis(2-nitrobenzoic acid) (DTNB) molecule [91]. Since AgNP surfaces have been functionalized with DTNB, NaCl-induced aggregations of AgNPs with a silica shell can become an efficient SERS system. These AgNPs-SiO2 structures have been functionalized with (3-aminopropyl) trimethoxy silane (APTMS) to form a primary amine layer on the surface. Subsequently, the surface of AgNPs-SiO2-NH2 structures has been immobilized with DEHP-COOH molecules by an amide binding through N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide (NHS) as a crosslinking reaction. DEHP aptamers have been immobilized on the surface of magnetic particles. There is a competitive  [83]. (C) Diagram of the fabricated photoelectrochemical aptasensors using AuNPs and ZnO nanopencils functionalized with BPA aptamers, which was aimed for BPA detection by means of the SPR technique. Adapted from [85].
binding between DEHP molecules and SERS silica particles with aptamer-coated magnetic particles. DEHP concentration has been quantitatively determined by comparing the signals of Raman reporter on SERS silica particles. Since DEHP has been widely used in plastic products, aptasensor-assisted SERS sensors can achieve highly selective detection of DEHP, despite interferents with analogous structures, which may provide numerous applications in food and environmental analysis. As summarized in Table 1, in the past decades, numerous novel optical sensors based on plasmonic resonance have emerged for indoor SVOC detection with improving aspects such as economical and reproducible benefits, as well as selectivity and sensitivity. The following points have been emphasized and evaluated on the current optical sensor methods. There are some perspectives that will be addressed in the near future such as: (1) continuing to improve optical sensor methods based on not only pure noble metallic substances but also hybrid nanostructured materials, (2) developing more aptasensors that exhibit highly selective specific sensors, (3) several novel nano-systems should be developed that can demonstrate the capability of multiple detections of analytically equivalent molecules under complex conditions, and (4) discovering a potential nanosystem used for simultaneous detection of both SERS and fluorescence. As summarized in Table 1, in the past decades, numerous novel optical sensors based on plasmonic resonance have emerged for indoor SVOC detection with improving aspects such as economical and reproducible benefits, as well as selectivity and sensitivity. The following points have been emphasized and evaluated on the current optical sensor methods. There are some perspectives that will be addressed in the near future such as: (1) continuing to improve optical sensor methods based on not only pure noble metallic substances but also hybrid nanostructured materials, (2) developing more aptasensors that exhibit highly selective specific sensors, (3) several novel nano-systems should be developed that can demonstrate the capability of multiple detections of analytically equivalent molecules under complex conditions, and (4) discovering a potential nanosystem used for simultaneous detection of both SERS and fluorescence.

Conclusions
Plasmonic resonance-based optical sensors have emerged as a powerful analytical tool that assists in optical detection of SVOC emission into environments. The effects of indoor SVOC exposure on human health through many pathways of dermal, inhalation, and non-dietary and dietary ingestion have been investigated in previous reports [12,13,55], and it still requires further studies using functional plasmonic nanomaterials and related detection methodologies. SVOCs as chemical additives have been widely used in many household products, inducing SVOC emission from resources to indoor air.
Effective sensing of indoor SVOCs using optical sensor methods require not only facile detection but also selective and rapid identification to warn of the early risks of the impact on human health. Both the novel nanoplatforms and aptasensors have been introduced to efficient optical sensors, including SERS, localized surface plasmon, and colorimetric sensor methodologies in a real indoor environment. Potential applications of plasmonic nanoplatforms and novel aptasensors have been discussed for optical detection of indoor SVOCs from surface-functionalized materials. Extensive studies of indoor SVOC detection are needed to maintain a clean, safe, and healthy environment. Functional plasmonic materials should be critical in optical detection of SVOCs in the future.