**1. Introduction**

Nanomaterial-based optical methods are advantageous for measuring quantitatively biomolecules of interest. With optical nanosensors, an extremely high photoluminescence intensity or enhancement effect in surface-enhanced Raman spectroscopy (SERS) can be achieved, and diversified signals, including wavelength/wavenumber, intensity, and excited-state lifetime, can be tuned to cater to specific applications. Most importantly, multiple signals can be integrated in a single optical nanoprobe to readily achieve ratiometric, multiplexed, and multimodality measurements. These detection modes are highly desirable for quantitative measurements in complex sample matrices [1,2].

Metal–organic frameworks (MOFs) are a kind of hybrid porous material consisting of inorganic metal ion or cluster nodes, and linkers including organic ligands and metal– organic complexes [3–7]. Due to the large specific surface area, the ultrahigh porosity, the adjustable internal surface property, the extraordinarily diversified structure, and the reasonable biocompatibility, MOFs are widely employed in storage and separation [8], catalysis, [9,10] drug delivery [5,11–15], and biomedicine [12,16–18], as well as in chemical sensors and biosensors [12,19–28]. In terms of optical measurements, MOFs are promising optical sensing materials because emission centers of MOFs can be constructed by "multiple photonic units" originating from inorganic metal ion or cluster nodes, linkers, or their combination to exemplify the features of structural diversity through combining inorganic and organic chemistry [29]. This unique property, together with tunable functional sites, imparts MOFs with highly designable and diversified luminescence that can be used for customized applications. On one hand, the "multiple photonic units" can be engineered

**Citation:** Wen, C.; Li, R.; Chang, X.; Li, N. Metal-Organic Frameworks-Based Optical Nanosensors for Analytical and Bioanalytical Applications. *Biosensors* **2023**, *13*, 128. https://doi.org/ 10.3390/bios13010128

Received: 13 December 2022 Revised: 9 January 2023 Accepted: 10 January 2023 Published: 12 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

or tailored with rational design to achieve the aforementioned diversified luminescence signals, all of which can be used for applications with specific needs; on the other hand, aside from attaching recognition entities for target-specific interaction, the functional sites can also be used to provide auxiliary interaction with emission centers to further tailor luminescence properties. Furthermore, taking the advantage of ultra-high porosity and modifiable internal surface property to encapsulate luminescent guest molecules in the porous structure, one more dimension of luminescence can be added to grant MOFs with ratiometric, multiplexing, and multimodality measurement capabilities [30,31]. In addition to luminescence, MOFs can serve as promising materials for SERS methods in which MOFs either act as substrates for signal enhancement or form composite substrates by encapsulating metallic nanomaterials. In addition to the above, the high surface area and controllable pore size allow a high capacity for adsorbing and concentrating analytes to achieve a low limit of detection (LOD) and a unique sieve effect, thus improving the selectivity [6,32].

There are quite a few review articles on MOFs and their applications in the optical measurement field [33–35]. This mini-review tries to highlight MOFs-based optical nanosensors, particularly luminescence and SERS, for applications in analytical chemistry. First, the application of MOFs in luminescence detection is summarized and discussed based on the evolution from a single luminophore, including MOFs itself or the encapsulated guest, to multiple signal sources for ratiometric and multimodality measurements. Second, the application of MOFs in SERS measurements is summarized and discussed based on the MOF itself as both a SERS substrate and a MOF-metal nanomaterial composite substrate. Finally, perspectives on future efforts to develop MOF-based nanosensors for analytical and bioanalytical applications are provided.

#### **2. MOFs-Based Nanosensors for Luminescence Applications**

Luminescence sensing has been recognized as an important tool in food safety detection, disease diagnosis, and environmental monitoring [36,37]. For these applications, it is required that the method has sufficient sensitivity and selectivity towards the analyte of interest. For analysis of targets in complex sample matrices, it is also desirable that the sensor or nanosensor system can self-calibrate to provide reproducible and quantitative results (Table 1).

Compared to conventional organic and inorganic luminophores, the luminescent MOF materials have excellent host and sensing features that can meet the aforementioned requirements for luminescence sensing [32,35,38]. The photoluminescence of MOFs may originate from metal centers, linkers, or guest molecules. Each emission unit can be designed and tuned for diversified application needs. Moreover, the integration of emissive units into MOFs can be adopted for the purpose of developing multi-emissive probes to realize self-calibrating measurements, multiplexed measurements, and multimodality measurements. The following discussion is based on the signal evolution in terms of the single luminophore signal, the ratiometric signal, and the multimodality signals. A perspective on multiplexed measurements for fluorescence applications will be discussed in the Conclusions and Future Perspectives section.

#### *2.1. Single Luminophore Signal*

The luminescence of MOFs may originate from the metal nodes [39], organic linkers [6], and guest molecules [40]. The luminescence may also originate from second building units (SBUs), including metal clusters [30,41,42] and metal–organic complex linkers [6,43], which are also used to build MOFs with extended porous networks and luminescence centers. All of the above can be utilized as luminescent probes to indicate the presence and even quantity of a tentative analyte [44]. The analyte can alter the luminescence of MOF sensors through one of the following mechanisms: intermolecular charge transfer, ligand-to-metal charge transfer (LMCT), photo-induced electron transfer (PET), Förster resonance energy transfer (FRET), dynamic quenching, or static quenching [45,46].

Lanthanide-based MOFs (Ln-MOFs), as metal node-based luminescence MOFs, present distinctive luminescence properties originating from abundant f-orbital configurations, long luminescence lifetimes, and "antenna effects" of linkers [47,48]. Qian and colleagues [39] used H3TATAB (4,4 ,4"-s-triazine-1,3,5-triyltri-*p*-aminobenzoic acid) as an organic linker to synthesize a series of isomorphic Ln-MOFs. For example, TbTATAB exhibited good stability in water and a high fluorescence quantum yield (77.48%). The large amount of N atoms afforded it a high ligand-Hg2+ affinity, which was able to block the antenna effect of the linker. As a result, the fluorescence of TbTATAB was quenched through either a dynamic or a static mechanism. The TbTATAB-based sensor showed excellent stability, reproducibility, and sensitivity, such that it could be practically employed to detect Hg2+ in environmental samples.

In addition to the metal nodes, the organic linker is another source of luminescence of MOFs. A luminescent MOF using aggregation-induced emission (AIE) molecules as an organic linker can confine the intramolecular motions of the linker molecule to give a bright emission [49]. Tang and colleagues [6] used an AIE linker, tetrakis(4-carboxyphenyl)ethylene (TCPE), to prepare ZnMOF and CoMOF with an unique [M+–L–M−–L–M]<sup>∞</sup> (M = metal clusters, L = linker) configuration (Figure 1). During the sensing of HCl vapor, ZnMOF exhibited a blue-toyellow-greenish transition of fluorescence due to the adsorption, rather than coordination, of HCl vapor. The adsorbed strong dipole HCl molecules were able to decrease the energy of the lowest singlet excited state via dipole–dipole interaction, leading to a red-shifted and weakened emission. For CoMOF, luminescence was quenched by cobalt ions. Introducing histidine could cause the collapse of the CoMOF framework and subsequent aggregation of TCPE to recover the blue emission of the TCPE aggregate.

**Figure 1.** HCl vapor sensing by ZnMOF. (**a**) Photographs of ZnMOF fumigated with HCl vapor from 12% HCl solution. (**b**) Fumigation time dependence. (**c**) HCl solution concentration dependence (fumigation time = 5 s) of solid-state fluorescence spectra of HCl-vapor fumigated ZnMOF. (**d**) Emission peak and intensity responses to HCl vapor concentration. (**e**) Schematics showing the solid-state fluorescence change of ZnMOF with adsorption and desorption of HCl vapor (the blue dotted line for host–guest interaction, and the black solid line for coordination), and the energy diagram of ZnMOF showing the influence of dipole–dipole interaction [6]. Copyright 2021, John Wiley and Sons.

Guest encapsulation into MOFs offers advantages over traditional synthesis of luminescent materials, such as easiness and cost effectiveness, as well as the possibility of tuning the emission properties by selection of guest molecules. [50] To date, various

luminescent molecules have been used as guests, including but not limited to Lanthanide (Ln) ions [47], quantum dots (QDs) [51,52], carbon dots (CDs) [40], upconversion nanoparticles (UCNPs) [3,11,16,53–59], noble metal nanoclusters [5,8,38,58,60–76], and organic dyes [20,77–82]. Encapsulation can prevent the aggregation-caused quenching (ACQ) of organic dyes and maintain the signal stability, the photostability, and the reasonable shelf-life of the fluorescent nanoparticles [30,38,77]. Wang and colleagues [40] reported a nanoscale complex based on CDs and MOFs (abbreviated as CDs@ZIF-8) for enhanced chemical sensing of quercetin. Quercetin can form a complex with CD in CDs@ZIF-8 via the electrostatic interaction between hydroxyl groups of quercetin and basic groups on the surface of CDs; thus, the fluorescence of CDs@ZIF-8 was quenched. Moreover, ZIF-8 endowed CDs@ZIF-8 with a high binding affinity to quercetin by π-π staking, as well as by improving the detection sensitivity and selectivity. The aforementioned characteristics of CDs@ZIF-8 guaranteed a LOD of 3.5 nM and its suitability for practical application in real samples for sensing of quercetin. In addition, Yan and colleagues [47] also used Ln ions as guest molecules to decorate MOF through post-synthetic modification in order to detect diphenyl phosphate in human urinary samples.

### *2.2. Ratiometric Signal*

Although single emission-based luminescent materials have been widely used, they suffer from inherent defects, such as signal fluctuations, variations of probe concentrations, light scattering from the matrix, signal fluctuations due to the complex matrices and sometimes sample pretreatments, etc. [30,77,79,83–86]. The ratiometric fluorescence sensing, based on the intensity ratio of two or more well-resolved emission bands, has the selfcalibrating capability to eliminate the aforementioned problems, and, thus, to enable more accurate measurements [80,84,87,88]. Due to the tunable multiple emission centers, the large absorption cross sections, and the tailorable skeletons [78,80], MOFs have been demonstrated to be a potential candidate for ratiometric sensing applications [78,79,83,84]. A ratiometric signal can be achieved between MOF and guest molecules [30,78–80,89], between guest and guest molecules [77], and even by MOF itself [32,42,84].

The tailor-made skeletons of MOFs offer specific host–guest interaction sites for intended recognition events. The guest molecules can be attached on the surface of MOFs through covalent linking [78] and electrostatic adsorption [40], or being encapsulated into the MOF channels [30,38,89]. Zhang and colleagues [78] synthesized nanoscale MOF (NMOF) for ratiometric peroxynitrite (ONOO−) sensing based on FRET (Figure 2). Poly(vinyl alcohol) (PVA) was used to attach the energy acceptor (ABt or BDP) on the surface of the energy donor (MOF). With 340-nm excitation of the donor, the nanosensor presented the emissions of the acceptors: 540 nm for ABt and 610 nm for BDP. The presence of ONOO− disabled FRET by detaching the acceptor from the donor. The quantification of ONOO− was realized based on the donor-to-acceptor intensity ratio. The fast response and high selectivity made the nanosensor suitable for imaging of ONOO− in living cells.

Yin and colleagues [89] reported a turn-on ratiometric fluorescent sensor, Ru@MIL-NH2, for water quantification. Ru(bpy)3 2+ was trapped in the channels of MIL-101(Al)-NH2 via a simple one-pot method. With the water content increasing from 0% to 100%, MIL-NH2 emission at 465 nm was intensified while the Ru(bpy)3 2+ emission remained stable at 615 nm with 300 nm excitation. It was revealed that the protonation of the nitrogen atom of the MIL-NH2, the π-conjugation system, and the stable fluorescence of Ru(bpy)3 2+ together facilitated the sensitive ratiometric measurements. This turn-on ratiometric fluorescence sensor showed low LOD (0.02%), fast response (less than 1 min), large dynamic range (0−100%), and good sensor reusability. The turn-on response is much simpler and more straightforward, and even more sensitive, than the quenching process [77].

**Figure 2.** The sensing and cell imaging of ONOO- with NMOF-Dye FRET pair. (**a**) Schematics of the sensing mechanism. (**b**) Fluorescence spectra of MA (1 mg L−1) as a function of ONOO<sup>−</sup> concentration in PBS buffer (pH 7.4) for 30 min; (**c**) fluorescence spectra of MA (1 mg L<sup>−</sup>1) with the addition of 1.00 mM ONOO− over time (0 to 20 min). (**d**,**e**) Fluorescence microscopic images of HeLa cells for exogenous ONOO−. (**d**) The cells were stained with MA (10 mg L<sup>−</sup>1) for 1 h and then washed with PBS before imaging; (**e**) the cells were pretreated as shown in (**d**) and then treated with SIN-1(1 mM) for 30 min. Scale bar: 25 mm [78]. Copyright 2017, Royal Society Chemistry.

A ratiometric signal can be achieved from the guest molecules and metal nodes of MOFs. Chen and colleagues [80] developed a MOF for real-time ratiometric fluorescent monitoring of food freshness by covalently coupling fluorescein 5-isothiocyanate (5-FITC) with NH2-rich EuMOF in a post-synthetic modification manner. Histamine, a biogenic amine produced by spoiled food, increased the emission of FITC at 525 nm and decreased the emission of Eu3+ at 611 nm. By doping the EuMOF-FITC probe on a flexible substrate (glass fiber), the complex was able to be integrated with a smartphone-based portable platform for on-site visual inspection of the freshness of raw fish samples.

MOFs with multi-emission centers are able to provide the ratiometric signal themselves. The metal-to-ligand and metal-to-metal energy transfers empower diversified luminescence responses. Shi and colleagues [32] synthesized a luminescent Eu-ZnMOF*n* by a structure engineering strategy, rendering the material enhanced slope sensitivity within the "optimized useful detection window" (Figure 3). Therefore, this biosensor enabled the discrimination of small concentration variations of urinary vanillylmandelic acid (VMA), an early pathological signature of pheochromocytoma. Upon the addition of VMA, emissions from organic linker at 433 nm became conspicuous with 330-nm excitation, while emissions from Eu3+ at 615 nm decreased. The organic linker emission change was attributed to the formation of an exciplex between the linker and VMA, which held a lower-lying excited-state energy level; the emission change in Eu3+ was due to the static quenching by VMA. This structure engineering strategy provided a facile approach to detect the biomarker change within a small concentration range, making the biosensor more suitable for clinical applications.

**Figure 3.** Ratiometric luminescence sensing of VMA by Eu-ZnMOF-*n*. (**a**,**b**) The widening of the excitation peak of Eu-ZnMOF-*n,* monitored at both 433 and 615 nm emissions with incremental addition of VMA. (**c**) The structure–property relationship of the probe's surface area to its dynamic range and sensitivity. (**d**) Color-coded digital array of Eu-ZnMOF-*n* probe after treatment with increased VMA concentrations under irradiation with a UV lamp (λex = 254 nm). [32] Copyright 2020, John Wiley and Sons.

The similar atomic radii and chemical properties of Ln ions make it feasible to prepare multi-emissive Ln-MOFs [87,90]. Previous studies have shown that a Tb-to-Eu energy transfer can occur [91]; therefore, the Tb/Eu mixed-Ln MOF can serve as the ratiometric sensor. Chen and colleagues [84] developed a mixed Ln-MOF luminescence thermometer using 2,5-dimethoxy-1,4-benzenedicarboxylate (DMBDC) as the linker. The Tb-to-Eu luminescent intensity ratio in Tb-DMBDC and Eu-DMBDC decreased as the temperature increased due to the thermal activation of non-radiative decay pathways. In the mixed-Ln-MOF, Eu0.0069Tb0.9931-DMBDC, the increment of temperature led to a decrease in Tb emissions and an increase in Eu emissions, thus forming a ratiometric fluorescence nanothermometer that presented a wide temperature dynamic range. The unique phenomenon can be attributed not only to the efficient antenna effect between DMBDC and Ln ions, but also to the Tb-to-Eu energy transfer.

Some non-luminescent MOFs, such as ZIF-8, can serve as host matrices to encapsulate guest organic dyes for ratiometric sensing. The confinement and isolation of the organic dyes can effectively inhibit the intramolecular torsional motion and increase the conformational rigidity to produce a high quantum yield [92]. Qian and colleagues [77] reported a luminescent nanothermometer by encapsulating luminescent dyes, 4-methylumbelliferone (4-Mu), and fluorescein (Flu) in the pores of ZIF-8. The developed nanothermometer can response to temperature changes based on the Flu-to-4-MU emission ratio (*I*Flu/*I*4-MU) as well as the emission peak wavelength of 4-MU. These two kinds of readouts can self-calibrate to ensure the accuracy of the detection. Furthermore, the nanosized property of ZIF-8 and the excellent luminescence properties of dyes impart the sensor with a large dynamic range and a high spatial resolution, which are important for temperature mapping [93].

In addition to fluorescence, phosphorescence can also be tuned by rationally designing the structure of metal nodes or organic linkers of MOFs [42,79]. MOFs can integrate both fluorescence and phosphorescence in one nanosensor. For example, the emission and lifetime of phosphorescence, but not fluorescence, are easily quenched by triplet oxygen, which can be utilized as a ratiometric signal based on the fluorescence-to-phosphorescence ratio. Lin and colleagues [79] designed the mixed-linker nanoscale UiO MOF and decorated the structure with Rhodamine-B isothiocyanate (RITC) to form R-UiO MOF for ratiometric sensing of intracellular O2 (Figure 4). In the nanostructure, the phosphorescent Pt-5,15-di(*p*benzoato)porphyrin (DBP-Pt) linker acted as an O2-sensitive probe, and the O2-insensitive fluorescent RITC served as a reference. With 514-nm laser excitation, emissions at 630 nm from DBP-Pt and 570 nm from RITC were observed. With the increase of O2 pressure, the DBP-Pt phosphorescence decreased significantly while the RITC fluorescence remained unchanged. The intracellular O2 of CT26 cells at 4, 32, and 160 mmHg was detected based on ratiometric signals using confocal laser scanning microscopy.

Zang and colleagues [42] reported a fluorescence–phosphorescence dual-emissive oxygen sensing MOF: ([Ag12(SBut )8(CF3COO)4(bpy-NH2)4]n (abbreviated as Ag12bpy-NH2)), based on silver–chalcogenolate-cluster and bipyridine (bpy) linkers. The introduction of the amino group enhanced the spin–orbit coupling and increased the intersystem crossing efficiency to boost triplet excitons and prolong the lifetime of phosphorescence at 556 nm in vacuum. As a result, oxygen molecules quenched the phosphorescence at 556 nm, while the fluorescence emission at 456 nm remained nearly invariant. This ratiometric quantification manner ensured a LOD as low as 0.1 ppm. The introduction of other substitutional groups, such as methyl or F− groups, extended the dynamic range of the ratiometric sensing. Therefore, tailoring the linker was deemed to be a powerful method for modulating luminescent sensing functionality.

Zhou and colleagues [94] used Prussian Blue (PB) and UCNPs to develop a nanoprobe (UC-PB) for the purpose of detecting and eliminating H2S with a linear range of 0–150 μM and an LOD of 50 nM. The Er-doped UCNP, NaLuF4:Yb,Er,Tm@NaLuF4, presented multiple emission peaks at 550 nm, 650 nm, and 800 nm, which were quenched by adding the PB shell. H2S triggered the decomposition of the PB shells to recover the strong upconversion luminescence (UCL) signal and the near infrared-to-green (N/G) ratio due to the cooperation of both redox and combination reactions. With the help of DL-propargylglycine (DL-PAG), the UC-PB was able to realize the *in vivo* near-infrared region ratiometric imaging, eliminate and inhibit the production of H2S, which is meaningful for clinical acute pancreatitis treatment.

**Figure 4.** Oxygen sensing based on the phosphorescence-to-fluorescence ratio of R-UiO-1. (**a**) Emission spectra (λex = 514 nm) and (**b**) phosphorescent decays (λex = 405 nm) of R-UiO-1 in HBSS buffer under varied oxygen partial pressures. Ratiometric luminescence imaging (λex = 514 nm) of CT26 cells after incubation with R-UiO-2 under (**c**) hypoxia, (**d**) normoxia, and (**e**) aerated conditions. Scale bar: 10 μm. (**f**) Schematics of fluorescence (Fl)–phosphorescence (Ph) dual emission centers, with Ph center quenched (Qu) in response to oxygen [79]. Copyright 2016, American Chemical Society.

#### *2.3. Multi-Modal Signal*

Fluorescence imaging (FLI), magnetic resonance imaging (MRI), and photoacoustic imaging (PAI) are widely used molecular imaging technologies which can realize noninvasive disease diagnosis and real-time *in vivo* lesion imaging [95,96]. Compared to single-modality imaging, which has the limitation of low penetration depth and low spatial resolution, multi-modality imaging integrates two or more modalities into one nanocomplex. Thus, it can provide more efficient and comprehensive information, which is desirable in the biomedical field [97].

Kuang and colleagues [53] developed an ultrasensitive and selective method for H2O2 detection based on a UCNP@ZIF-8/NiSx chiral complex. The NiSx moiety is a chiral nanoparticle with circular dichroism (CD) signals at 440 and 530 nm. The presence of the NiSx can quench the UCL signal of the UCNPs core at 540 nm, with the UCL signal at 660 nm remaining unchanged. The introduction of H2O2 caused NiSx to degrade, accompanied by the recovery of the UCL signal and the disappearance of the CD signal. This dual-mode signal of CD and fluorescence changes opens a new avenue for developing a toolbox for biomedical and biological analyses.

Yin and colleagues [98] reported a MnO2-coated, hollow mixed metal (Mn/Cu/Zn) MOF that could allow the photosensitizer, indocyanine green (ICG), to have a high loading efficiency. The coexistence of Cu<sup>+</sup> and Cu2+ in the MOF, as verified by X-ray photoelectron spectroscopy, endows the complex with a glutathione-responsive "turn on" MRI ability. With the laser irradiation, the ICG can serve not only as the fluorescence and photothermal imaging agent, but also as the photodynamic therapy (PDT) agent. Therefore, this hollow MOF was used as a trimodality imaging-guided tumor therapy agent to highlight the efficiency of mixed-metal and mixed-valence strategies in tumor theranostic capacities.

For small functional molecule encapsulation, the risk of leakage and burst release is always a challenge [99]. To solve the aforementioned problem, Yang and colleagues [100] used the one-pot approach to prepare a Fe-based MOF (MIL-53) with defect structure due to the introduction of near-infrared dye (cypate). Further decoration of PEG and transferrin on the surface of nanoparticles (denoted as CMNP-Tf) was able to accelerate the passive and active targeting to the tumor region. The presence of cypate endowed the nanoparticles with excellent PAI and near-infrared fluorescence (NIRF) imaging properties, as well as reactive oxygen species (ROS) generation and photothermal therapy abilities. Furthermore, Fe also possesses a *T*1-weighted MRI contrast property. Therefore, the CMNP-Tf can realize the NIRF-, PAI-, and MRI-guided tumor targeting imaging-guided photothermal/photodynamic performance.


**Table 1.** Summary of MOF based fluorescence applications.

#### **3. MOFs-Based Nanosensors in SERS Applications**

Surface-enhanced Raman spectroscopy is a hypersensitive technique that enhances Raman scattering of the analyte in proximity to a nanostructured substrate via electromagnetic or chemical enhancement [4,105]. It can provide structural fingerprint information on the low-concentration analyte in real time [106,107]. Owing to its high sensitivity and high selectivity, SERS has a breadth of applications in pharmaceutical and environmental analysis [69,108], food science [109], life sciences [5], clinical diagnosis [64,69], and other fields [106]. However, for metal substrates, problems remain regarding the application of real-world sample detection. To name a few, the selective adsorption of analytes onto the metal substrate is required to assure better sensitivity and specificity [110]; the binding strength between the analyte and plasmonic surfaces needs to be improved to facilitate better chemical enhancement; and the stability and reproducibility of metal substrates need to be improved to assure the robustness of the analytical methods [105].

As a new class of porous polymeric materials, MOFs present ultra-high porosity, a large surface area, and designable binding sites for multiple functionalization, which provide concentrating effects and multiple selectivity for the analytes [22,23,63]. In addition to directly serving as the SERS substrate, the MOF-metal composite SERS substrate can further improve both the sensitivity and the selectivity of the analytical measurements; in such a way, the stability and uniformity of the enhancement substrate can be improved [32,75]. Therefore, MOF materials have found increasing applications in SERS measurements (Scheme 1, Table 2).

**Scheme 1.** Applications of MOF-based surface-enhanced Raman spectroscopy (SERS).



**Table 2.** *Cont.*

a: Chemical enhancement; b: electromagnetic enhancement.

#### *3.1. SERS Substrates*

To date, many MOFs have been directly used as SERS substrates; examples include ZIF-67, ZIF-8, Mo-MOF, MIL-100(Fe), etc. [91,112,114]. The enhancement by MOF can be attributed to chemical enhancement (CM) [63,105], which has been proposed as the primary enhancement mechanism for plasmon-free substrates such as MOF, semiconductors, and other metal oxides (Cu2O [73], WO3 [115], TiO2 [116], VO2 [117–119]), etc. Particularly, charge transfer transitions may be major contributors to SERS [120]. The charge transfers between the highest occupied molecular orbital (HOMO) of the analyte and the conduction band (CB) edge of the substrate material, or between the valence band (VB) of the substrate material and the lowest unoccupied molecular orbital (LUMO) of the molecules, are crucial for the enhancement effects. Other than the resonance Raman enhancement due to electronic transition between HOMO and LUMO, the electronic transition between VB and CB of the substrate material can also contribute to the enhancement of the Raman signal due to the resonance process [121–123].

The first MOF-enhanced Raman spectroscopic study was conducted by Tsung-Han Yu et al. [121] using methyl orange (MO) adsorbed in MIL-100 and MIL-101 as the model system. The study suggested that the Raman intensity enhancement was due to the charge transfer between the metal oxide clusters in MOFs and the adsorbed MO molecules. The study also suggested that the SERS effect was also orientation-dependent, which is in agreement with the basic understanding of Raman spectroscopy.

MIL-100(Fe) has been demonstrated, for the first time, by Li and colleagues to act as a SERS-active substrate to detect volatile organic compounds (VOCs) that usually possess low Raman cross-sections [112], and a LOD of 2.5 ppm was achieved for toluene. Based on special adsorption energy and density functional theory (DFT) calculations, the charge transfer enhancement mechanism was suggested for high SERS activity. Selective enhancement was attributed to resonance between laser energy and the photo-induced charge transfer energy, as well as the different dispersive energy between the ligand of the MOFs and the analyte (Figure 5). Through the ion exchange strategy, the MIL-100(Fe-Zr) complex improved LOD for isopropanol (50 ppm) compared to the pristine MIL-100(Fe) (100 ppm). This MIL-100(Fe)-based sensing platform was successfully used to monitor gaseous indicators, including 4-ethylbenzaldehyde, acetone, and isopropanol, for early diagnosis of lung cancer.

**Figure 5.** SERS of gaseous small molecules by MIL-100 (Fe). (**a**) Energy-level diagrams of toluene, acetone, and chloroform relative to MIL-100 (Fe) with respect to the vacuum level. (**b**) Adsorption affinity and binding energy of toluene, acetone, and chloroform on MIL-100(Fe). (**c**) SERS spectra of toluene, acetone, and chloroform vapor, respectively, on MIL-100(Fe), with concentrations as indicated [112]. Copyright 2020, John Wiley and Sons.

MOFs have a high degree of tailorability, with the ability to choose nodes and linkers as well as to tune the framework topologies. Consequently, the electronic energy band structure can be tuned to match the molecular orbital energy level of the analyte in order to facilitate controllable combinations of several resonances, such as the charge transfer, interband, and molecule resonances, in addition to the ground-state charge transfer interactions; thus, a significantly enhanced Raman signal can be achieved. Encouragingly, based on the pore-structure optimization and surface modification strategy, Zhao and colleagues synthesized a series of MOFs by using metal clusters M2(COO)4 (M = Zn, Co, and Cu) as

the nodes and Tetrakis(4-carboxyphenyl)porphyrin (TCPP) or 2-methylimidazole as the linker (Figure 6) [114]. The electronic band structure of the MOFs can be tuned to match that of the target analyte, such that the enhancement factor (EF) of ZIF-67 can reach as high as 1.9 × <sup>10</sup><sup>6</sup> with the LOD as low as 10−<sup>8</sup> M. The high flexibility of the MOF structure can provide high levels of variety for SERS applications.

**Figure 6.** Structure and SERS effect of M-TCPP MOF. (**a**) Illustration of the process of synthesizing the M-TCPP (M = Zn, Co, and Cu) MOFs. (**b**,**c**) SEM images of the Cu-TCPP MOFs and Zn-TCPP MOFs. (**d**) PXRD patterns of the Co-TCPP MOFs, Zn-TCPP MOFs, and Cu-TCPP MOFs. (**e**) SERS spectra of R6G (10−<sup>4</sup> M) on the Co-TCPP, Zn-TCPP, and Cu-TCPP substrates. (**f**) SERS spectra of R6G on the Co-TCPP substrate at three different concentrations: 10<sup>−</sup>4, 10−5, and 10−<sup>6</sup> M [114]. Copyright 2019, American Chemical Society.

Using MOFs as the SERS-active substrate can realize selective enhancement of the target molecule for the Raman signal. However, precise tuning of the band energy is required to facilitate resonance with laser energy and charge transfer energies, which makes it difficult to realize SERS for a breadth of molecules of interest using one MOF substrate. Furthermore, there is still room to improve the sensitivity which can be achieved by the combination of MOFs with plasmonic substrates.

Electromagnetic enhancement (EM) is a physical enhancement process attributed to the localized surface plasmon resonance (LSPR) of the noble metal [4]. When the laser impinges on the metal nanostructure, e.g., a metal nanoparticle, the electromagnetic wave causes the collective oscillation of the delocalized conduction electrons. LSPR occurs when the frequency of the light matches with the oscillation frequency. The coupling of incident light with the metal nanostructures results in a huge enhancement of the local electromagnetic field, which is the major mechanism accounting for SERS. The combination of MOF with noble metal SERS substrates may be able to realize the synergetic enhancement by integrating CM and EM into one system, thus achieving maximum sensitivity [23,63]. Furthermore, such a MOF-SERS combination may be but one solution for problems associated with metal substrates, such as the low concentration and high mobility of the analyte at the enhancement spot, complicated matrix interferences, and modest substrate stability [63,124]. Specifically, the adjustable pore size of MOFs enables them to serve as molecular sieves to filter the analyte based on molecular size [71,75,104]; MOFs' shells can protect the metal substrates from oxidation and reactive species in the complex matrix, thus improving the stability [125].

MOFs coatings have the advantage of controlling the hotspot distribution to improve the SERS performance [71–73]. In the SERS technique, the Raman signal can be significantly enhanced only when the analyte is confined to the proximal distance to the plasmonic surface (<3–5 nm), because enhancement depends on the exponential decay of the plasmonic field at the metal–medium interface [63]. In the work by Wang and colleagues, gold superparticles (GSPs) were used to provide high-density hotspots, and the coating of ZIF-8 over GSPs provided a further enhanced SERS effect [64] (Figure 7). The finite-difference time-domain (FDTD) calculation revealed that the MOF coating was able to enhance the intensity of the electromagnetic field around the metal surface, and the high dielectric constant of MOF prevents the decay of the electromagnetic field along the radial direction. Consequently, a very intense SERS effect was observed [64,75].

**Figure 7.** GSPs@ZIF-8 and the mechanism for selective SERS detection. (**a**) Hotspots in GSPs (left) and GSPs@ZIF-8 (right) around the edges of the plasmon. (**b**) SERS spectra of 4-ATP on GSPs (black) and GSPs@ZIF-8 (red), with the inset showing the relative intensity of the Raman spectra. (**c**) An illustration of the principle for achieving the selectivity. (**d**) The maximum diameter of four main analytes that were allowed to pass through the pores of ZIF-8. (**e**) The reaction used to capture aldehyde vapors via covalent linkage to the GSPs [64]. Copyright 2018, John Wiley and Sons.

The synergistic SERS effect was achieved by Wang and colleagues, via site-selective deposition of ZIF-8 on gold nanobipyramids (Au NBPs) [71]. Deposition of ZIF-8 was observed around the distal end, waist, or the surface of the Au NBPs (Figure 8). When ZIF-8 was located at the distal ends (Au BNPs@end-ZIF 8), the largest electric field enhancement was achieved, and the Raman signals on Au BNPs@end-ZIF 8 were at least twice those on the Au NBPs. Li and colleagues used ZIF-8-coated cuprous oxide/silica core–shell nanostructure (Cu2O@SiO2@ZIF-8) as a template to precisely control the growth of Ag NPs of varying sizes (2–29 nm) [73]. The results reflected that when the size of the Ag NPs matched well with the pores of Cu2O@SiO2@ZIF-8, the strongest electromagnetic field was generated. The Cu2O@SiO2@ZIF-8 provided abundant and uniformly distributed hotspots, thus resulting in an LOD of 5.76 × <sup>10</sup>−<sup>12</sup> mol·L−<sup>1</sup> and a limit of quantification (LOQ) of 1.92 × <sup>10</sup>−<sup>12</sup> mol·L<sup>−</sup>1, respectively, in the detection of phenol red.

**Figure 8.** SERS of NBP@ZIF. (**a**) Schematics showing the routes for selective deposition of ZIF-8 on the Au NBPs and SERS detection. (**b**) SERS spectra measured with the ZIF-8 nanocrystals, Au NBPs, NBP/end-ZIF, NBP/waist-ZIF, and NBP@ZIF nanostructures for the detection of aniline and 4-ATP, respectively [71]. Copyright 2021, American Chemical Society.
