Periodic Reporting for period 1 - MIRASPEC (Miniature on-chip Raman spectrometer for personal volatile organic compound (VOC) monitoring)
Reporting period: 2018-01-01 to 2019-06-30
Summary of the context and overall objectives of the project
Volatile organic compounds (VOCs) are organic chemicals with a high vapor pressure at room temperature. VOCs are ubiquitous in indoor air as they are emitted by a multitude of sources, both anthropogenic and natural. Common indoor VOC sources include wall paint, furniture polish, and burnings of candles. However, many VOCs can cause both acute and chronic adverse health effects [1,2]. Several institutes, including World Health Organization (WHO) Environmental Protection Agency in the US (EPA), and French Indoor Air Quality Observatory (OQAI) have established guidelines on VOC concentrations [3–5]. Driven by the increasing awareness of environmental protection and personal safety, there is a growing demand for portable devices for continuous on-site VOC monitoring with low cost. However, current methods for VOC monitoring are largely unsatisfactory, as they are based on labor-intensive sampled intervention with results available only once every few hours.
In the ERC-POC project MIRASPEC, we have proposed an innovative VOC monitoring device based on waveguide-enhanced Raman spectroscopy (WERS). Probing directly the molecular fingerprints, Raman spectroscopy delivers an analytical prowess beyond other gas sensing techniques. It can identify and quantify chemical components in a complicated matrix unambiguously with minimal sample preparation, allowing the continuous and in-situ detection of VOC. However, it has been the major hurdle so far to extend Raman spectroscopy to ultrasensitive VOC sensing applications.
This hurdle can be overcome by WERS [6] using functionalized silicon nitride waveguides where the evanescent tail of the guided mode is used to excite and collect the Raman signal efficiently. Two enhancement mechanisms are leveraged on WERS to allow for ppm-level VOC sensing. On the one hand, both the optical excitation and the analyte are tightly confined within the vicinity of silicon nitride waveguides, which greatly enlarges the light-matter interaction volume, leading to significant enhancement in signal strength [7]. On the other hand, we introduce a layer of mesoporous material on top of the waveguide to adsorb VOCs and enrich their concentration. In collaboration with the Technische Universität Wien, we have developed a mesoporous coating capable of enriching VOCs by 24000 times, leading to greatly enhanced signals.
As a result of these two benefits, we have demonstrated, to the best of our knowledge, the first Raman sensor for broadband vapor-phase VOC. The sensor exhibits high sensitivity. As a proof-of-principle demonstration, we have tested the sensor with three VOC gases: acetone, ethanol, and isopropyl alcohol (IPA). Their limit of detection is determined to be 594 ppm, 157 ppm, and 53 ppm respectively with a 1 second measurement time. Further optimizations in waveguide length, waveguide cross-section and chemical properties of the mesoporous coating can improve the sensitivity down to sub-10 ppm level. We believe this work constitutes a significant step forward towards an all-on-a-chip Raman sensor suitable for environmental monitoring and clinical diagnosis [8].
To demonstrate a fully integrated Raman system suitable for indoor VOC monitoring, three other components, in addition to the waveguide sensor, need to be integrated: a pump-rejection filter, a spectrometer, and a light source. In previous years, we have already demonstrated a novel on-chip Fourier Transform Spectrometer (FTS) and the integration of III-V light sources at 1550nm on a silicon platform through transfer printing technologies. Therefore, this ERC-POC project has focused on the design, fabrication, and optimization of a pump-rejection filter. Raman spectroscopy requires a high extinction ratio for the pump laser to avoid saturation of the detectors. However, most of the demonstrations can obtain only a 40dB extinction due to fabrication imperfections. Within this project, we have demonstrated an innovative design with cascaded grating-assisted contra-directional couplers. This approach can efficiently suppress parasitic reflections caused by fabrication imperfection, and we have demonstrated an extinction ratio of 68.5 dB [9].
The work in MIRASPEC has demonstrated that WERS is a promising candidate for low-concentration VOC sensor in indoor air quality monitoring. The functionalized Raman sensor and high-performance components have been investigated within this project. We have shown that low-concentration VOC sensing is indeed possible, as demonstrated with three VOC gases. Using a standard CMOS process toolset for fabricating the Raman spectrometer, the waveguide-based Raman sensor can be produced with high cost-efficiency and high yield, at a price level at least an order of magnitude lower than current commercial systems. However, more research is still needed to implement this innovation into products. The sensitivity of the current VOC sensor is still below the requirement, and full integration with laser, sensor, pump-rejection filter and spectrometer is yet to be demonstrated. The difficulty of these challenges, however, is largely manageable, and these problems can be overcome within a reasonable amount of time. It is noteworthy that the mesoporous functionalized waveguide is indeed a versatile platform that can be extended to multiple applications, including biomedical sensing in bodily fluids.
1. L. Massolo, M. Rehwagen, A. Porta, A. Ronco, O. Herbarth, and A. Mueller, "Indoor–outdoor distribution and risk assessment of volatile organic compounds in the atmosphere of industrial and urban areas," Environ. Toxicol. 25, 339–349 (2010).
2. T. Petry, D. Vitale, F. J. Joachim, B. Smith, L. Cruse, R. Mascarenhas, S. Schneider, and M. Singal, "Human health risk evaluation of selected VOC, SVOC and particulate emissions from scented candles," Regul. Toxicol. Pharmacol. 69, 55–70 (2014).
3. World Health Organization, WHO Guidelines for Indoor Air Quality: Selected Pollutants (2010).
4. K. Dimitrios, K. Kimmo, K. Stylianos, C. Paolo, M. Marco, S. Christian, J. Matti, C. Christian, K. Severine, L. Thomas, M. James, and M. Lars, "The INDEX Project - Critical Appraisal of the Setting and Implementation of Indoor Exposure Limits in the EU," (2005).
5. "État de la qualité de l' air dan les logements français, rapport final," http://www.oqai.fr/userdata/documents/Document_133.pdf(opens in new window).
6. A. Dhakal, A. Z. Subramanian, P. Wuytens, F. Peyskens, N. Le Thomas, and R. Baets, "Evanescent excitation and collection of spontaneous Raman spectra using silicon nitride nanophotonic waveguides," Opt. Lett. 39(13), 4025–4028 (2014).
7. G. E. Walrafen and J. Stone, "Intensification of Spontaneous Raman Spectra by Use of Liquid Core Optical Fibers," Appl. Spectrosc. 26, 585–589 (1972).
8 H. Zhao, B. Baumgartner, A. Raza, A. Skirtach, B. Lendl, and R. Baets, "Multiplex Volatile Organic Compound (VOC) Raman Sensing With Nanophotonic Slot Waveguides Functionalized With A Mesoporous Enrichment Layer," submitted for publication.
9. X. Nie, N. Turk, Y. Li, Z. Liu, and R. Baets, "High extinction ratio on-chip pump-rejection filter based on cascaded grating-assisted contra-directional couplers in silicon nitride rib waveguides," Opt. Lett. 44, 2310–2313 (2019).
In the ERC-POC project MIRASPEC, we have proposed an innovative VOC monitoring device based on waveguide-enhanced Raman spectroscopy (WERS). Probing directly the molecular fingerprints, Raman spectroscopy delivers an analytical prowess beyond other gas sensing techniques. It can identify and quantify chemical components in a complicated matrix unambiguously with minimal sample preparation, allowing the continuous and in-situ detection of VOC. However, it has been the major hurdle so far to extend Raman spectroscopy to ultrasensitive VOC sensing applications.
This hurdle can be overcome by WERS [6] using functionalized silicon nitride waveguides where the evanescent tail of the guided mode is used to excite and collect the Raman signal efficiently. Two enhancement mechanisms are leveraged on WERS to allow for ppm-level VOC sensing. On the one hand, both the optical excitation and the analyte are tightly confined within the vicinity of silicon nitride waveguides, which greatly enlarges the light-matter interaction volume, leading to significant enhancement in signal strength [7]. On the other hand, we introduce a layer of mesoporous material on top of the waveguide to adsorb VOCs and enrich their concentration. In collaboration with the Technische Universität Wien, we have developed a mesoporous coating capable of enriching VOCs by 24000 times, leading to greatly enhanced signals.
As a result of these two benefits, we have demonstrated, to the best of our knowledge, the first Raman sensor for broadband vapor-phase VOC. The sensor exhibits high sensitivity. As a proof-of-principle demonstration, we have tested the sensor with three VOC gases: acetone, ethanol, and isopropyl alcohol (IPA). Their limit of detection is determined to be 594 ppm, 157 ppm, and 53 ppm respectively with a 1 second measurement time. Further optimizations in waveguide length, waveguide cross-section and chemical properties of the mesoporous coating can improve the sensitivity down to sub-10 ppm level. We believe this work constitutes a significant step forward towards an all-on-a-chip Raman sensor suitable for environmental monitoring and clinical diagnosis [8].
To demonstrate a fully integrated Raman system suitable for indoor VOC monitoring, three other components, in addition to the waveguide sensor, need to be integrated: a pump-rejection filter, a spectrometer, and a light source. In previous years, we have already demonstrated a novel on-chip Fourier Transform Spectrometer (FTS) and the integration of III-V light sources at 1550nm on a silicon platform through transfer printing technologies. Therefore, this ERC-POC project has focused on the design, fabrication, and optimization of a pump-rejection filter. Raman spectroscopy requires a high extinction ratio for the pump laser to avoid saturation of the detectors. However, most of the demonstrations can obtain only a 40dB extinction due to fabrication imperfections. Within this project, we have demonstrated an innovative design with cascaded grating-assisted contra-directional couplers. This approach can efficiently suppress parasitic reflections caused by fabrication imperfection, and we have demonstrated an extinction ratio of 68.5 dB [9].
The work in MIRASPEC has demonstrated that WERS is a promising candidate for low-concentration VOC sensor in indoor air quality monitoring. The functionalized Raman sensor and high-performance components have been investigated within this project. We have shown that low-concentration VOC sensing is indeed possible, as demonstrated with three VOC gases. Using a standard CMOS process toolset for fabricating the Raman spectrometer, the waveguide-based Raman sensor can be produced with high cost-efficiency and high yield, at a price level at least an order of magnitude lower than current commercial systems. However, more research is still needed to implement this innovation into products. The sensitivity of the current VOC sensor is still below the requirement, and full integration with laser, sensor, pump-rejection filter and spectrometer is yet to be demonstrated. The difficulty of these challenges, however, is largely manageable, and these problems can be overcome within a reasonable amount of time. It is noteworthy that the mesoporous functionalized waveguide is indeed a versatile platform that can be extended to multiple applications, including biomedical sensing in bodily fluids.
1. L. Massolo, M. Rehwagen, A. Porta, A. Ronco, O. Herbarth, and A. Mueller, "Indoor–outdoor distribution and risk assessment of volatile organic compounds in the atmosphere of industrial and urban areas," Environ. Toxicol. 25, 339–349 (2010).
2. T. Petry, D. Vitale, F. J. Joachim, B. Smith, L. Cruse, R. Mascarenhas, S. Schneider, and M. Singal, "Human health risk evaluation of selected VOC, SVOC and particulate emissions from scented candles," Regul. Toxicol. Pharmacol. 69, 55–70 (2014).
3. World Health Organization, WHO Guidelines for Indoor Air Quality: Selected Pollutants (2010).
4. K. Dimitrios, K. Kimmo, K. Stylianos, C. Paolo, M. Marco, S. Christian, J. Matti, C. Christian, K. Severine, L. Thomas, M. James, and M. Lars, "The INDEX Project - Critical Appraisal of the Setting and Implementation of Indoor Exposure Limits in the EU," (2005).
5. "État de la qualité de l' air dan les logements français, rapport final," http://www.oqai.fr/userdata/documents/Document_133.pdf(opens in new window).
6. A. Dhakal, A. Z. Subramanian, P. Wuytens, F. Peyskens, N. Le Thomas, and R. Baets, "Evanescent excitation and collection of spontaneous Raman spectra using silicon nitride nanophotonic waveguides," Opt. Lett. 39(13), 4025–4028 (2014).
7. G. E. Walrafen and J. Stone, "Intensification of Spontaneous Raman Spectra by Use of Liquid Core Optical Fibers," Appl. Spectrosc. 26, 585–589 (1972).
8 H. Zhao, B. Baumgartner, A. Raza, A. Skirtach, B. Lendl, and R. Baets, "Multiplex Volatile Organic Compound (VOC) Raman Sensing With Nanophotonic Slot Waveguides Functionalized With A Mesoporous Enrichment Layer," submitted for publication.
9. X. Nie, N. Turk, Y. Li, Z. Liu, and R. Baets, "High extinction ratio on-chip pump-rejection filter based on cascaded grating-assisted contra-directional couplers in silicon nitride rib waveguides," Opt. Lett. 44, 2310–2313 (2019).