Front. Photonics Frontiers in Photonics Front. Photonics 2673-6853 Frontiers Media S.A. 1213434 10.3389/fphot.2023.1213434 Photonics Mini Review On-chip liquid sensing using mid-IR plasmonics Hinkov et al. 10.3389/fphot.2023.1213434 Hinkov B. 1 * David M. 1 Strasser G. 1 Schwarz B. 1 Lendl B. 2 1 Institute of Solid State Electronics and Center for Micro- and Nanostructures, Technische Universität (TU) Wien, Vienna, Austria 2 Institute of Chemical Technologies and Analytics, Technische Universität (TU) Wien, Vienna, Austria

Edited by: Michele Ortolani, Sapienza University of Rome, Italy

Reviewed by: Tommaso Giovannini, Scuola Normale Superiore, Italy

 *Correspondence: B. Hinkov, borislav.hinkov@tuwien.ac.at 27 06 2023 2023 4 1213434 27 04 2023 08 06 2023 Copyright © 2023 Hinkov, David, Strasser, Schwarz and Lendl. 2023 Hinkov, David, Strasser, Schwarz and Lendl

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The investigation of molecules in the mid-IR spectral range has revolutionized our understanding in many fields such as atmospheric chemistry and environmental sensing for climate research or disease monitoring in medical diagnosis. While the mid-IR analysis of gas-samples is already a mature discipline, the spectroscopy of liquids is still in its infancy. However, it is a rapidly developing field of research, set to fundamentally change our knowledge of dynamical processes of molecules in liquid-phase. In this field, mid-IR plasmonics has emerged as breakthrough concept for miniaturization, enabling highly-sensitive and -selective liquid measurement tools. In this review, we give an overview over current trends and recent developments in the field of mid-IR spectroscopy of molecules in liquid phase. Special attention is given to plasmon-enhanced concepts that allow measurements in highly compact sensor schemes. Nowadays, they reach full monolithic integration, including laser, interaction section and detector on the same chip, demonstrating unprecedented operation in situ and real-time analysis of chemical processes.

 mid-infrared plasmonics lab-on-a-chip liquid sensing bio-sensing proteins in situ quantum cascade laser optoelectronics Horizon 2020 Framework Programme10.13039/100010661 European Research Council10.13039/501100000781 section-at-acceptance Plasmonics 1 Introduction

Semiconductors have transformed our everyday life in a variety of different ways (Bardeen and Brattain, 1948; Hall et al., 1962; Huang et al., 2020; Fraunhofer-ISE, 2022; Huang et al., 2022). They are known to be fundamental components of computers and mobile phones, but nowadays also enter in other fields when being implemented into fridges and baking ovens as remotely controllable parts in the “internet of things.” In particular, semiconductor-based optoelectronics is a field of compact devices for the conversion of electrical into optical signals like in LEDs (Cho et al., 2017; Huang et al., 2020) and (diode) lasers (Hall et al., 1962; Faist et al., 1994; Yang, 1995), or vice versa, for generating electrical signals from measuring photons in detectors and imaging instruments (Broudy and Mazurczyk, 1981; Levine et al., 1987; Hofstetter et al., 2002; Yang et al., 2010). While for decades optoelectronic devices have already been the backbone of our data transmission and telecommunication infrastructure (Nadiri and Nandi, 2003; Dely et al., 2022; Flannigan et al., 2022; Pang et al., 2022; Submarine Communication, 2023), they are becoming increasingly relevant in molecular spectroscopy in recent years (Curl et al., 2010; Celebrano et al., 2011; Haas and Mizaikoff, 2016; Hinkov et al., 2022).

 2 Light sources in the mid-IR spectral range

The mid-IR spectral range is the part of the electromagnetic spectrum, hosting the fundamental vibrational “fingerprint” absorptions of many molecules (Li et al., 2013; Schwaighofer and Lendl, 2020; CFA, 2023). For their detection, they are typically analyzed using thermal- or laser-based light sources. The former often use globars (Yashunsky et al., 2010; Haas and Mizaikoff, 2016) (a SiC-rod heated to ∼1,250°C) in Fourier-transform infrared (FTIR-)spectrometers, being able to obtain full mid-IR spectra from 400 to 4,000 cm−1 in a single-shot measurement on the seconds-to-few-minutes time-scale (Baker et al., 2014; Baumgartner et al., 2018; De Meutter and Goormaghtigh, 2021; Schwaighofer et al., 2021; Szwarcman et al., 2021). However, their major drawback is a very low emission power per wavelength in the μW/cm−1 range (Brandstetter et al., 2010; Schwaighofer and Lendl, 2020), which is a particular issue in liquids. On the contrary, the probably most widely used mid-IR lasers are the quantum cascade laser (QCL) that was first demonstrated by Faist et al. (1994) and the interband cascade laser (ICL) that was realized for the first time by Yang et al., in 1995 (Yang, 1995). QCLs exploit tailored intersubband transitions in quantum wells, allowing to design their emission wavelength by bandstructure engineering (Faist, 2013) from ∼3–12 μm (Bai et al., 2011; Lyakh et al., 2012a; Bismuto et al., 2012; Hinkov et al., 2013; Schwarz et al., 2017). In contrast, ICLs use a type-II band alignment active region based on tailorable interband transitions and show strong performance in the range of ∼2.8–6 μm wavelength (Vurgaftman et al., 2013; Scheuermann et al., 2015). Today, both, QCLs and ICLs, are highly-reliable and versatile mid-IR laser light sources with room-temperature (RT) and continuous-wave (CW) operation (Bai et al., 2011; Lyakh et al., 2012b; Hinkov et al., 2012; Vurgaftman et al., 2013; Weih et al., 2014; Schwarz et al., 2017; Knötig et al., 2020; Meyer et al., 2020). State-of-the-art devices emit up to ∼6-9 orders of magnitude higher spectral power densities (= W-kW/cm−1) (Vurgaftman et al., 2013; Schwaighofer and Lendl, 2020) than globars, and they can be further scaled up by using very narrow linewidth singlemode devices based on distributed feedback (DFB) gratings (Bartalini et al., 2011; Tombez et al., 2012). The much narrower spectral coverage of mid-IR laser emission as compared to globars can be significantly increased by using widely tunable external-cavity (EC) lasers (Wysocki et al., 2005; Hinkov et al., 2009; Hugi et al., 2009; Fuchs et al., 2010; Riedi et al., 2013), DFB devices (Faist et al., 1997; Lu et al., 2011; Xie et al., 2012; Suess et al., 2016; Hinkov et al., 2019) extended to multi-wavelengths array geometries (Mujagić et al., 2011; Rauter et al., 2013; Jouy et al., 2015; Süess et al., 2016; Marschick et al., 2023) or frequency comb configurations (Villares et al., 2014; Consolino et al., 2020; Sterczewski et al., 2020; Komagata et al., 2023). One important additional feature of those mid-IR lasers relevant for miniaturization towards chip-scale applications, is their ability to be used as QC detectors (QCDs) (Hofstetter et al., 2002) or IC infrared photodetectors (ICIPs) (Li et al., 2005; Yang et al., 2010), respectively. QCDs are typically operated unbiased (Hofstetter et al., 2002; Reininger et al., 2013; Delga, 2020; Marschick et al., 2022), show low dark current detection (Delga, 2020; Marschick et al., 2022), similar to QCLs, GHz-bandwidth operation (Hinkov et al., 2016; Dely et al., 2022) and a large range of linear response, even at high power levels (Dabrowska et al., 2022; Marschick et al., 2022). It is important to note, that QC devices are ideal candidates for integration with plasmonic concepts, since they inherently support TM-polarization only (Faist, 2013; Jollivet et al., 2018; Delga, 2020). This enables direct excitation of surface plasmon polaritons (SPPs) in suitable surface geometries.

 3 Mid-IR spectroscopy

The mid-IR spectral range hosts many important applications such as sensing of environmental greenhouse gases (Kosterev et al., 2008; Tuzson et al., 2008; EPA, 2014; IPCC, 2022), pharmaceutical analysis and production techniques as well as petrochemical applications (ASTM D6304 – 16, 2021; Garcia-Perez et al., 2008; Ricchiuti et al., 2022; Pilat et al., 2023), point-of-care medical diagnosis including in situ bio-medical analysis and wearables (Pleitez Rafael et al., 2013; Baldassarre et al., 2016; Lu et al., 2020; Smuck et al., 2021), spectral imaging (Amrania et al., 2018; Kilgus et al., 2018; Razeghi, 2020) and security applications (Pushkarsky et al., 2006; Fuchs et al., 2010; Hinkov et al., 2010). In addition, it is rapidly unlocked for optical free-space communication with Gbit s−1 transmission rates (Dely et al., 2022; Flannigan et al., 2022; Pang et al., 2022) in the spectral windows of low atmospheric attenuation between 3–5 μm and 8–12 μm wavelength (Flannigan et al., 2022; CFA, 2023). Mid-IR spectroscopy analyzes molecules in gas (Curl et al., 2010; Patimisco et al., 2014; Haas and Mizaikoff, 2016; Schwaighofer et al., 2017; Szedlak et al., 2018; Hinkov et al., 2019; Waclawek et al., 2019), liquid (Murayama and Tomida, 2004; Barth, 2007; Barreca et al., 2010; De La Arada et al., 2012; Amenabar et al., 2013; Mizaikoff, 2013; Pleitez Rafael et al., 2013; Lu et al., 2015; Rodrigo et al., 2015; Güler et al., 2016; Schwaighofer et al., 2016; Bibikova et al., 2017; Barelli et al., 2020; Chowdhury et al., 2020; Norahan et al., 2021; Szwarcman et al., 2021) and solid phase (Fuchs et al., 2010; Hinkov et al., 2010; Celebrano et al., 2011; Amrania et al., 2012; Amrania et al., 2018). Gas-sensing is probably the most developed field among them, addressing the very narrow absorption lines of gas-molecules ( < 1 cm−1) (Li et al., 2013; Tuzson et al., 2013; CFA, 2023). This finds application in many fields including isotope spectroscopy (Kerstel, 2003; Bartlome and Sigrist, 2009; Li et al., 2013; Wunderlin et al., 2013) of CO2 (Tuzson et al., 2013; Van Geldern et al., 2014; Wang et al., 2017) for emission source identification based on monitoring isotope-resolved concentration patterns. In the past 2 decades, numerous high-sensitivity and -selectivity gas-sensing techniques have been developed, based on direct absorption spectroscopy following the Beer-Lambert absorption law (Barth, 2007; Tuzson et al., 2013). Due to the large absorption length (distance into the medium for 1/e-signal-attenuation) when measuring in gases, they often rely on increasing the effective path length in the gas analyte up to the meter-scale, e.g., by multi-reflection gas cells which simultaneously also decrease the sensor footprint (Li et al., 2013; Tuzson et al., 2013). Instead, alternative advanced spectroscopic schemes measure other quantities and thus can implement specific capabilities. Prominent examples are: i) the baseline-free chirped laser dispersion spectroscopy (CLaDS), probing the refractive index change in a gas, ii) (quartz-enhanced) photoacoustic spectroscopy (QEPAS) (Kosterev et al., 2008; Patimisco et al., 2018; Ma et al., 2022) that analyzes a generated periodic acoustic wave with a (quartz-)tuning fork and can be calibration-free (Wu et al., 2017), iii) balanced interferometric cavity assisted photothermal spectroscopy (B-ICAPS) (Waclawek et al., 2016; Waclawek et al., 2019), which again probes refractive index changes, but this time through a generated photothermal signal, and which scales linearly with gas concentration and is suitable for sensor miniaturization (Waclawek et al., 2016) and iv) dual-comb spectroscopy with mid-IR QCL (Villares et al., 2014; Consolino et al., 2020; Komagata et al., 2023) or ICL frequency combs (Sterczewski et al., 2020), which relies on analyzing a heterodyne beating signal. In contrast to gas-phase spectroscopy, detecting molecules in liquid-phase needs to address broad absorption features (≫50 cm−1) in a much denser medium (Schwaighofer and Lendl, 2020). While the former demands for broadband sensors, the latter results in orders of magnitude lower absorption lengths. Typical corresponding penetration lengths in highly absorbing aqueous solutions are a few micrometers only for thermal-light-source-based sensors like FTIR-spectrometers (Fabian and Mäntele, 2006; Haas and Mizaikoff, 2016; Schwaighofer and Lendl, 2020; Dabrowska et al., 2022) and can reach up to tens of micrometers (Schwaighofer et al., 2016; Akhgar et al., 2020; Schwaighofer et al., 2021) and above (Schwarz et al., 2014; Hinkov et al., 2022) for laser-based techniques. It can be further significantly increased by using a low-absorbing matrix, e.g., D2O instead of H2O for protein analysis in the amide I band (Murayama and Tomida, 2004; Barreca et al., 2010; De La Arada et al., 2012; Lu et al., 2015; Yang et al., 2015; Güler et al., 2016; Strazdaite et al., 2020; De Meutter and Goormaghtigh, 2021).

 4 Protein-sensing with discrete optical components

While FTIR-based liquid sensing approaches are currently getting more and more substituted or complemented by laser-based techniques, state-of-the-art measurement and analysis tools are still often using tabletop geometries with discrete components. Protein-sensing in the mid-IR is a field of research of high relevance for pharmaceutical and bio-medical applications (Baldassarre et al., 2016; Schwaighofer et al., 2017; Kumar et al., 2018; Shrivastav et al., 2021; Altug et al., 2022) with a rich body of existing literature (Barth, 2007; Baldassarre et al., 2016; López-Lorente et al., 2017; Kumar et al., 2018; Shrivastav et al., 2021; Szwarcman et al., 2021; Altug et al., 2022). It will act as prototype-field in this review paper for discussing typical discrete-component measurement systems, including for the analysis of e. g., poly-l-lysine (PLL) (Schwaighofer and Lendl, 2020; Mousavi et al., 2021), bovine serum albumin (BSA) (Murayama and Tomida, 2004; Barreca et al., 2010; Lu et al., 2015; Güler et al., 2016; Schwaighofer et al., 2016; De Meutter and Goormaghtigh, 2021; Hinkov et al., 2022), α-Chymotrypsin (Yang et al., 2015) or the milk proteins β-lactoglobulin, α-lactalbumin and casein (Dabrowska et al., 2022). Those proteins are traditionally analyzed in the “protein fingerprint region”, the amide I band between 1,600–1700 cm−1, which mainly arises from their C=O stretching vibration with some other minor contributions (Barth and Zscherp, 2002). Measuring proteins in the mid-IR enables access to their structural properties, such as the protein secondary structure, which are essential for protein function (Murayama and Tomida, 2004; Lu et al., 2015; Yang et al., 2015; Güler et al., 2016; Schwaighofer et al., 2016; De Meutter and Goormaghtigh, 2021; Hinkov et al., 2022). Those properties were recently exploited by Schwaighofer et al. (2016), who analyzed the thermal denaturation of the secondary structure of the polypeptide PLL in the amide I range with an EC-QCL, a Mercury cadmium telluride (MCT-)detector and a temperature-controlled flow cell. Using deuterated solution allowed a film thickness of 478 μm for monitoring concentrations of 0.25–10 mg mL−1 under controlled pH-conditions. Lu et al. (Lu et al., 2015) investigated the thermal denaturation of BSA in D2O buffer, identifying two different temperature ranges (50°C–52°C and 80°C–82°C) for protein structure changes. They used a FTIR-MCT setup and a flow cell equipped with an ATR-based silver-halide fiber sensor for 290 μm films. Yang et al. (Yang et al., 2015) published a FTIR-based routine for analyzing the protein secondary structure of e.g., α-Chymotrypsin and other proteins at high concentrations above 3 mg mL−1 in aqueous solution (H2O and D2O). And Dabrowska et al. (Dabrowska et al., 2022) analyze the bovine milk proteins β-lactoglobulin, α-lactalbumin and casein in a broadband EC-QCL-QCD setup covering a spectral range above 260 cm−1 for concentrations of 0.25–15 mg mL−1 and a film thickness of 12.5 μm. Multivariate sample analysis of protein mixtures using the partial least square (PLS) method, allows identifying individual constituents at high figures-of-merit (R2 > 0.98).

 5 Compact liquid sensing schemes based on mid-IR plasmonics

While FTIR- and laser-based techniques have revolutionized the field of mid-IR liquid sensing, their often rather bulky experimental geometries do not allow sensing on rapid time-scales or even in situ sample analysis. A wide range of novel and suitable approaches targets this issue by miniaturized mid-IR sensors based on the exploitation of plasmonic concepts (Homola, 2006; Biagioni et al., 2012; Rodrigo et al., 2015; Neubrech et al., 2017; Taliercio and Biagioni, 2019; Barelli et al., 2020; Altug et al., 2022; Hinkov et al., 2022). SPPs are collective oscillations of the electron density at the intersection of two materials with sign change of the real part of their electrical permittivity, such as at a metal-dielectric interface (Sarid, 1981). From their dispersion relation, the condition for the permittivity ϵ of both materials for successful SPP excitation and propagation can be derived to be: ϵ m > ϵ d (m: metal and d: dielectric permittivity) (Law et al., 2013). Corresponding, highly confined sub-wavelength plasmonic modes (Barnes et al., 2003; Ozbay, 2006; Falk et al., 2009) enable high-speed photonic properties and below-diffraction-limit device miniaturization for visible (Guo et al., 2013; Huang and Luo, 2018; Sistani et al., 2019) to near-IR wavelengths (Huang and Luo, 2018; Sistani et al., 2019), with electrical wire (Falk et al., 2009; Guo et al., 2013) to optical waveguide geometries (Ozbay, 2006; Lal et al., 2007; Guo et al., 2013; David et al., 2021). While strong confinement to a metallic surface yields high guiding losses and limited propagation lengths (Guo et al., 2013), still high-performance and high-speed SPP as well as localized surface plasmon (LSP) detectors can be realized (Huang and Luo, 2018; Sistani et al., 2019; Sistani et al., 2020). LSPs are the localized counterpart of SPPs, typically excited in metallic nanostructures (Law et al., 2013; Bibikova et al., 2017; López-Lorente et al., 2017).

5.1 Recent developments in mid-IR plasmonics

The above described SPP and LSP concepts work very well for UV to near-IR wavelengths based on the use of (noble or transition) metals (Au, Ag, Ni, Cu (Aroca et al., 2004; Law et al., 2013; Perry et al., 2013)) with their plasma-frequencies in the deep-UV to visible range. The situation is completely different in the mid-IR spectral range. The permittivity ϵ m is much larger (also than ϵ d ), yielding a much higher plasmonic mode extension into the dielectric, e.g., ∼3x to > 10x the modal wavelength for an Ag/air interface (Law et al., 2013). This results in a significantly larger mode propagation length on the order of above 1,000x wavelengths (mm-scale) (Law et al., 2013). Unfortunately, it is not always useful for mid-IR liquid-sensing applications, where coupling to active or passive on-chip components on the wavelength-scale is needed. For avoiding those limitations of metal-based mid-IR plasmonics, lower-plasma-frequency materials have been demonstrated, including highly-doped epitaxial semiconductors (Taliercio and Biagioni, 2019; Ehlers and Mills, 1987; Gómez Rivas et al., 2004; Ginn et al., 2011; Law et al., 2012; N’tsame Guilengui et al., 2012; Augel et al., 2016; Frigerio et al., 2016; Pellegrini et al., 2018), such as Ge, Si, III-Vs (e.g., GaP and GaN) or II-VIs (Barker, 1968; Harima et al., 1998; Streyer et al., 2014; Zhong et al., 2015; Taliercio and Biagioni, 2019), transparent conductive oxides (Zhong et al., 2015; Castellano, 2022), silicides (Soref et al., 2008; Naik et al., 2013; Zhong et al., 2015), transition metal nitrides (Zhong et al., 2015) and graphene (Fei et al., 2012; Grigorenko et al., 2012; Zhong et al., 2015; Constant et al., 2016). However, they mainly fit to Si-photonics integration or to implementation into CMOS-structures and lack simple fabrication and implementation protocols, compatible with mid-IR technology. As an alternative approach, structured-metal “spoof” SPP geometries are an interesting option (Pendry et al., 2004; Williams et al., 2008; Yu et al., 2008) that has been used to pattern QCL-facets to collimate their output beam in mid-IR (Yu et al., 2008) or THz devices (Yu et al., 2010). Lately, the concept of combining dielectric loading (DL) with noble metal plasmonics is generating significant interest. It has previously been used at telecom wavelengths (Holmgaard and Bozhevolnyi, 2007; Steinberger et al., 2007; Kumar et al., 2013; Krasavin and Zayats, 2015) and shows similarities to hybrid plasmonic concepts (Nielsen et al., 2014; Zhang et al., 2017). At near-IR wavelengths, DLSPP waveguides have been used because of their: i) direct control over the trade-off between mode confinement and propagation length enabling complex plasmonic circuits and ii) their flexibility to use dielectric materials with particular thermo- or electro-optic properties (Kumar et al., 2013). In the mid-IR, DLSSP waveguides increase the vertical mode confinement significantly, allowing the realization of complex mid-IR photonic integrated circuits (PICs). By simply adding a ∼200–300 nm thick slab of SiN (Schwarz et al., 2014) or Ge (David et al., 2021) to a ∼100–200 nm thick Au layer for ∼6.5–9.5 μm wavelength, the resulting SPP mode becomes vertically confined to the wavelength-scale, while maintaining up-to mm-scale propagation lengths (Schwarz et al., 2014; David et al., 2021). Since SiN absorbs above 7 μm (Kischkat et al., 2012), Ge can be a suitable alternative that is transparent in the whole mid-IR between 2 μm and 14 μm and that can be used in a similar way as dielectrics in DLSPPs, in so-called semiconductor loaded SPP (SLSPP) waveguides (David et al., 2021).

 5.2 Plasmonic sensing concepts in the mid-IR

Highly-sensitive and -selective liquid-phase spectroscopy using compact metal-dielectric structures has been a well-established field for near-UV to near-IR wavelengths (Sreekanth et al., 2016). It enables overcoming diffraction limitations of conventional chip-scale approaches (Amenabar et al., 2013; Kilgus et al., 2018). For momentum mismatch compensation when coupling an external light source to such a SPP surface, mode coupling (Raether, 1988; Barnes et al., 2003) and control mechanisms (Raether, 1988; Yu et al., 2010; Thongrattanasiri et al., 2011) were introduced, by using external prisms (Otto or Kretschmann configuration) (Sreekanth et al., 2016; Castellano, 2022), by implementing high-index layers (Law et al., 2013) or by spoof SPP geometries (Pendry et al., 2004; Yu et al., 2010; Kushiyama et al., 2012; Law et al., 2013). In contrast, LSPs do not need momentum matching because of their tunability of the resonance frequency (Sreekanth et al., 2016) through altering the plasmonic particle shape or by modifying its dielectric environment (Law et al., 2013; Bibikova et al., 2017; López-Lorente et al., 2017). In the mid-IR, LSPs cannot be directly excited in sub-wavelength spheres and particles, which act as close to perfect conductors in this wavelength range ( ϵ m → ∞ ) and do not support plasmonic mode penetration into and coupling to their metallic surface. Again, materials with lower plasma frequency can be used (Taliercio and Biagioni, 2019; Ginn et al., 2011; Law et al., 2012; N’tsame Guilengui et al., 2012; Augel et al., 2016; Frigerio et al., 2016; Pellegrini et al., 2018; Zhong et al., 2015). Figure 1 shows a selection of metal-based mid-IR liquid sensor concepts, which are mainly relying on plasmonic enhancement (Figures 1B–D). Infrared reflection absorption spectroscopy (IRAS) (Figure 1A) is an early approach using molecules on homogeneous metal films and the only displayed non-plasmonic technique (Hoffman, 1983). Surface-enhanced IR absorption spectroscopy (SEIRA) (Figure 1B) (Osawa, 1997; Aroca et al., 2004) is probably the most widely used plasmonic sensing technique, where the light is coupled to LSPs on metal islands or nanostructures (Law et al., 2013). SEIRA supports local near-field molecular absorption enhancement with broadband spectral resonances, enabled by the random metal roughness of the surface islands. It was first demonstrated in 1980 by Hartstein et al. (1980) in silver nanoparticles and shares similarities with surface-enhanced Raman spectroscopy (SERS) (Fleischmann et al., 1974; Nie and Emory, 1997; Langer et al., 2020). It includes comparable signal values, even though both techniques show very different absorption enhancements of 10–1,000 (SEIRA) and 1014-1015 (SERS) (Nie and Emory, 1997). SERS is beneficial for localized short-range molecule analysis, while SEIRA has advantages in probing thicker films. Prism-coupled SEIRA (Kretschmann or Otto configuration) (Figure 1C) combines localized field enhancement with SPP characteristics in patterned metal films (Hatta et al., 1984). It has also been used exploiting strong SPP enhancement in biological applications (Golosovsky et al., 2009). Finally, surface-plasmon-enhanced infrared absorption (SPEIRA) (Figure 1D) relies on exploiting the effect of extraordinary optical transmission (EOT) in metallic grating structures, e.g., in Au, Ag, Cu, Ni (Ebbesen et al., 1998; Martín-Moreno et al., 2001; Williams and Coe, 2006; Wasserman et al., 2007; Liu and Lalanne, 2008). Its enhancement factor is about x100 as compared to IRAS, resulting from a longer SPP-path length. Based on these plasmonic concepts, different compact liquid sensors have been realized. One often used geometry is based on attenuated total reflection (ATR) in surface-coated semiconductor crystals (e.g., Si or Ge) (Bibikova et al., 2017; López-Lorente et al., 2017; Wacht et al., 2022). In the work by Bibikova et al. (2017) and López-Lorente et al. (2017) nanoparticle-based resonant enhancement on top of Si-ATR-crystals was achieved by either using spherical gold nanoparticles and anisotropic gold nanostars for measuring thioglycolic acid (enhancement: 10x) or BSA (enhancement: 2x) in H20 (Bibikova et al., 2017) or by using Au nanoparticles with BSA (enhancement: 2x) in H20 and D20 (López-Lorente et al., 2017). Another complementary SEIRA approach by Yoo et al. (2018) exploits a wafer scale array of zeroth-FP-order resonant coaxial nanoapertures with 7 nm gap size which analyzes 5 nm thick silk protein films. The result is an impressive absorption enhancement factor of 104–105. Additional work on SEIRA spectroscopy is summarized in the following two review papers: (Neubrech et al., 2017; Shrivastav et al., 2021). Neubrech et al. (2017) review the field of “resonant SEIRA”, i.e., resonant metal nanoantennas including their underlying physics and routes for maximizing SEIRA enhancement based on the used geometry, arrangement and material. For more work on mid-IR plasmonic nanoantennas we refer to (Biagioni et al., 2012; Baldassarre et al., 2015; Celebrano et al., 2015; Celebrano et al., 2021; Di Francescantonio et al., 2022). The review by Shrivastav et al. (2021) gives an overview over current plasmonic-based biosensors for viral diagnostics based on SEIRA, propagating/localized surface-plasmon resonance (SPR) and SERS. Returning to plasmonic sensor concepts based on ATR geometries, Baumgartner et al. (Baumgartner et al., 2018; Baumgartner et al., 2019), Wacht et al. (2022) and Frank et al. (2021) show the functionalization of ATR-crystals for significantly enhanced sensitivity, by using Silica- (Baumgartner et al., 2018; Baumgartner et al., 2019), Zirconia- (Wacht et al., 2022) and Titania-based (Frank et al., 2021) ordered mesoporous films, respectively. Typically achieved enrichment factors lie on the order of > 200 (benzonitrile, silica film), > 100 (valeronitrile, silica film) and 162 (benzonitrile, Zirconia film) including the possibility to modify the surface into a hydrophobic state for repelling water. Finally, a wide variety of other plasmonic (bio-)sensors have been realized. Rodrigo et al. demonstrate a tunable nanostructured graphene biosensor for label-free protein monolayer detection (Rodrigo et al., 2015). Similar protein monolayers were investigated by Wu et al., exploiting multipixel arrays of Fano-resonant asymmetric metamaterials (FRAMMs) (Wu et al., 2012). More work on Fano-resonances in nanoscale plasmonic geometries can e.g., be found by Giannini et al. (2011). The field of nanophotonic biosensors using evanescent-field sensing in plasmonic metal-resonances and Mie resonances in dielectrics for label-free detection was recently reviewed by Altug et al. (2022). Kumar et al. (2018) instead give a review of the field of novel biosensor platforms for water-borne pathogen analysis using e.g., SPR concepts and more.

Schematics of various plasmonic sensing techniques: (A) IRAS. (B) Reflection-geometry SEIRA. (C) Kretschmann-configuration SEIRA for coupling through an external prism. (D) SPEIRA. Figure reprinted from Law et al. (2013), licensed under CC BY 3.0 with permission from the authors.

 5.3 Mid-IR liquid sensing on the chip-scale

The previously discussed concepts demonstrate impressive results with respect to sensor specificity, sensitivity and in parts to compactness. Still, the resulting setups are regularly rather bulky with external (laser) light sources and thus still often yield time consuming offline measurements. This poses a strong limitation for applications in the analysis of dynamical processes in liquids such as chemical reactions (Norahan et al., 2021). The full monolithic integration of QCL, DL-plasmonic interaction section and QCD into a lab-on-a-chip sensor is a breakthrough solution that was realized by Schwarz et al. (2014); Ristanic et al. (2015) and recently used for in situ real-time monitoring of BSA (by Hinkov et al.) (Hinkov et al., 2022) and of an organic solvent by Pilat et al. (2023). It is summarized in Figure 2. Most recent work shows, that the plasmonic waveguides can be further improved, including: i) increased bandwidth in Ge-SLSPPs covering a full octave between 5.6–11.2 μm wavelength (David et al., 2021), ii) implementation of surface passivation coatings for protection from damaging liquids (David et al., 2023a), iii) surface functionalization for chemically specific enrichment and improved sensing of liquids (David et al., 2023a) and iv) on-chip plasmonic mode guiding based on novel polymeric materials like polyethylene (David et al., 2022; David et al., 2023b).

Overview over the application of the fully monolithic QC-technology based lab-on-a-chip concept. (A) Linear lab-on-a-chip concept based on QC technology and DLSPP waveguides. (B) DLSPP waveguide FEM-simulations for a SiN/Au configuration with the commercial software Comsol Multiphysics 5.5: (left) mode profile in air and (right) vertical coupling and DLSPP profile in air or D2O. (C) (left) Design of a 60-μL fluid cell. (right) Picture of a submersion sensor configuration: the sensor chip is the golden square soldered in the middle of the white PCB. (D) Thermal denaturation spectra of the model protein BSA investigated between 1,575 and 1700 cm−1 and for temperatures between 50°C and 90°C with an ATR-FTIR sensor. (E) Contact angle measurements of water drops on a non-activated (top, hydrophilic, 3 μL, ϑ <90°) and activated (bottom, hydrophobic, 7 μL, ϑ >90°) ZrO2-coated SLSPP waveguide surface. Figure (A) Adapted with permission from Schwarz et al., Nature Communications 5, 4,085, 2014; DOI: https://doi.org/10.1038/ncomms5085; licensed under CC BY-NC-SA 3.0. Figures (B) and (D) reproduced under CC-BY 4.0, Hinkov et al. (2022). Figure (C) reproduced from Pilat et al.(2023), licensed under CC BY 4.0 with permission from the Royal Society of Chemistry. Figure (E) reproduced under CC-BY 4.0, from arXiv2305.16522 [physics.optics].

 6 Discussion

Future developments in the field of plasmon-enhanced mid-IR liquid sensing are expected to further pursue chip-scale concepts. Particular current work in this field includes the realization of much more complex mid-IR PICs and photonic networks by implementing mode guiding and beam manipulating capabilities, similar to near-IR photonics (Soref, 2006). This will allow a much better beam steering control in the mid-IR as observed in free-space geometries (Hinkov et al., 2008). The novel on-chip concepts will potentially enable highly-sensitive plasmonic on-chip interferometers, e.g., in a “Mach-Zehnder” configuration or other heterodyne concepts which strongly benefit from miniaturized sensors. Furthermore, the implementation of plasmonic structures allowing single-molecule detection (Celebrano et al., 2011) or of microfluidic capabilities through polymer-based, on-chip structures, will additionally boost the use of such monolithic liquid sensors (Schwarz et al., 2014; Hinkov et al., 2022). The implementation of those new capabilities will open the pathway towards real-life sensing applications in disease monitoring, such as measuring specific protein-marker configurations as early diagnostic indicators for Parkinson’s disease and other health conditions that can be monitored through body-fluid analysis. This can go as far as including in vivo bio-sensing applications (Pleitez et al., 2013; Pleitez Rafael et al., 2013) and enable the realization of the next-generation of commercial sensors based on fully integrated fingertip-sized geometries.

 Author contributions

BH wrote the manuscript with editorial input from MD, GS, BS, and BL. All authors contributed to technical discussions and commented on the paper. All authors contributed to the article and approved the submitted version.

 Funding

BH, MD and GS received funding from the EU Horizon 2020 Framework Program (project cFlow, No. 828893). BH acknowledges funding by the Austrian Science Fund FWF (M2485-N34). BH, GS and BL acknowledge financial support from the EU Horizion 2020 Framework Program (project REDFINCH, No. 780240). BS received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant agreement No. 853014). BL acknowledges financial support from the European Union’s research and innovation programme Horizon 2020 and Horizon Europe (projects AQUARIUS, No. 731465; HYDROPTICS, No. 71529; M3NIR, No. 101093008; BROMEDIR, No. 101092697).

Fruitful discussions with H. Detz, F. Pilat, W. Schrenk and E. Gornik and expert technical assistance by A. Linzer are greatly acknowledged.

 Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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ATR attenuated total reflection
BI-CAPS balanced detection cavity-assisted photothermal spectroscopy
BSA bovine serum albumin
CW continuous-wave
DFB distributed feedback
DL dielectric loading
EC external-cavity
EOT extraordinary optical transmission
FRAMM Fano-resonant asymmetric metamaterial
FTIR Fourier-transform infrared
ICIP interband cascade infrared photodetector
ICL interband cascade laser
IRAS infrared reflection absorption spectroscopy
LED light emitting diode
LSP localized surface plasmon
MCT mercury cadmium telluride
mid-IR mid-infrared
near-IR near-infrared
PIC photonic integrated circuit
PLL poly-l-lysine
PLS partial least square
RT room temperature
QCD quantum cascade detector
QCL quantum cascade laser
QEPAS quartz-enhanced photoacoustic spectroscopy
SEIRA surface-enhanced IR absorption spectroscopy
SERS surface-enhanced Raman spectroscopy
SL semiconductor loading
SPEIRA surface-plasmon-enhanced IR absorption
SPP surface plasmon polariton
SPR surface-plasmon resonance
TM transverse magnetic