B. Hinkov
M. David
G. Strasser1
B. Lendl- 1Institute of Solid State Electronics and Center for Micro- and Nanostructures, Technische Universität (TU) Wien, Vienna, Austria
- 2Institute of Chemical Technologies and Analytics, Technische Universität (TU) Wien, Vienna, Austria
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.
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 (
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:
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
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
FIGURE 1. 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).
FIGURE 2. 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).
Acknowledgments
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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Glossary
Keywords: mid-infrared plasmonics, lab-on-a-chip, liquid sensing, bio-sensing, proteins, in situ, quantum cascade laser, optoelectronics
Citation: Hinkov B, David M, Strasser G, Schwarz B and Lendl B (2023) On-chip liquid sensing using mid-IR plasmonics. Front. Photonics 4:1213434. doi: 10.3389/fphot.2023.1213434
Received: 27 April 2023; Accepted: 08 June 2023;
Published: 27 June 2023.
Edited by:
Michele Ortolani, Sapienza University of Rome, ItalyReviewed by:
Tommaso Giovannini, Scuola Normale Superiore, ItalyCopyright © 2023 Hinkov, David, Strasser, Schwarz and Lendl. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: B. Hinkov, Ym9yaXNsYXYuaGlua292QHR1d2llbi5hYy5hdA==