Spectroscopic sensors of liquid represent a fundamental tool in different research areas as for instance biology, chemistry, forensics, pharmaceutical research and environmental monitoring. The persistent efforts towards the miniaturization of such devices are driven by two fundamental advantages: the sensor portability which makes it possible its use for in situ applications, and the ability to strongly limit the use of large amounts of solutions and reagents which are necessary to perform the measurements. In this paper, an on-chip implementation of jet waveguide based optofluidic spectroscopic detection scheme is reported. The adopted detection scheme exploits total internal reflection arising in a microjet leading to highly efficient signal collection in fluorescence spectroscopy [1]. The same approach can be used also for Raman spectroscopy [2]. More specifically, the sensor is designed so that the water jet ejecting from a capillary is intercepted from a 7 optical fiber probe in circular arrangement. Each fiber has a diameter of 50?m. The central fiber is used to excite the analyte present in the microjet waveguide produced by means of a micro-pump. An alternative choice is an orthogonal excitation of the liquid waveguide. The surrounding 6 fiber of the bundle are used to collect the signal in both of the cases. The micro-pump provides a flow rate of 250 ml/hr. This value has been chosen to ensures adequate stability of the jet and it corresponds to a breakup length (i.e. the maximum length of the jet possible at this specific liquid velocity) around 10mm, which is superior to the distance between the optical probe and the capillary nozzle (7mm) i.e. the jet portion exploited as optical waveguide. This high flow rate allows to horizontally arrange the sensor. The alignment between the microjet and the optical fiber probe is provided by means of a frame composed by two layers of Polymethylmethacrylate (PMMA). These layers are suitably micro-machined to house the capillary and the fiber probe. This results in a microfluidic chip with totally alignment-free spectroscopic detection scheme, which avoids any background from the sample containers. The schematic of the microjet waveguide on chip sensor is shown in figure 1. As it is necessary to shield the sensor from external light, otherwise present in the detected signal, a housing structure of the chip has been expressly designed and fabricated by means of a 3D printing system. But the most important feature of this housing structure of the chip is related to the fact of being able to provide a recirculation of the solution under analysis. For this reason, within the housing structure, it is present an inner reservoir which is connected to the inlet tube of the micro-pump. As fluorescent dyes are commonly used to label cells, proteins, nucleic acids and antibodies. [3], in order to apply the on-chip spectroscopic sensor on fluorescence spectroscopy, measurements of eosin Y in water solutions have been considered. A diode laser emitting at 532nm with a power of 10 mW has been used as excitation source, whereas a mini-spectrometer connected to a laptop has been used as detector. The limit of detection, determined with eosin Y solution, has been 38 pM by employing an integration time of only 40 ms. As the typical concentration ranges considered in fluorescence labelling is often several order of magnitudes higher than the one explored in this work, the proposed approach appears as a valid strategy in fluorescence spectroscopy. For instance an optimum concentration of eosin B for protein estimation was worked out to be 0.01% in [4], whereas more limited quantity of eosin Y (10-7 mol/l) are necessary in an eosin Y-based fluorescent sensor which is used for detection of perfluorooctane sulfonate [5]. Due to the versatility of the design, the system is appropriate also for Raman spectroscopy. In this case, probe excitation configuration, exploiting the microjet waveguide capability also for the excitation represents a valid strategy due to the well-known weak sensitivity offered by Raman spectroscopy.
MICROJET WAVEGUIDE ON-CHIP SENSOR FOR SPECTROSCOPY
G Persichetti;G Testa;R Bernini
2017
Abstract
Spectroscopic sensors of liquid represent a fundamental tool in different research areas as for instance biology, chemistry, forensics, pharmaceutical research and environmental monitoring. The persistent efforts towards the miniaturization of such devices are driven by two fundamental advantages: the sensor portability which makes it possible its use for in situ applications, and the ability to strongly limit the use of large amounts of solutions and reagents which are necessary to perform the measurements. In this paper, an on-chip implementation of jet waveguide based optofluidic spectroscopic detection scheme is reported. The adopted detection scheme exploits total internal reflection arising in a microjet leading to highly efficient signal collection in fluorescence spectroscopy [1]. The same approach can be used also for Raman spectroscopy [2]. More specifically, the sensor is designed so that the water jet ejecting from a capillary is intercepted from a 7 optical fiber probe in circular arrangement. Each fiber has a diameter of 50?m. The central fiber is used to excite the analyte present in the microjet waveguide produced by means of a micro-pump. An alternative choice is an orthogonal excitation of the liquid waveguide. The surrounding 6 fiber of the bundle are used to collect the signal in both of the cases. The micro-pump provides a flow rate of 250 ml/hr. This value has been chosen to ensures adequate stability of the jet and it corresponds to a breakup length (i.e. the maximum length of the jet possible at this specific liquid velocity) around 10mm, which is superior to the distance between the optical probe and the capillary nozzle (7mm) i.e. the jet portion exploited as optical waveguide. This high flow rate allows to horizontally arrange the sensor. The alignment between the microjet and the optical fiber probe is provided by means of a frame composed by two layers of Polymethylmethacrylate (PMMA). These layers are suitably micro-machined to house the capillary and the fiber probe. This results in a microfluidic chip with totally alignment-free spectroscopic detection scheme, which avoids any background from the sample containers. The schematic of the microjet waveguide on chip sensor is shown in figure 1. As it is necessary to shield the sensor from external light, otherwise present in the detected signal, a housing structure of the chip has been expressly designed and fabricated by means of a 3D printing system. But the most important feature of this housing structure of the chip is related to the fact of being able to provide a recirculation of the solution under analysis. For this reason, within the housing structure, it is present an inner reservoir which is connected to the inlet tube of the micro-pump. As fluorescent dyes are commonly used to label cells, proteins, nucleic acids and antibodies. [3], in order to apply the on-chip spectroscopic sensor on fluorescence spectroscopy, measurements of eosin Y in water solutions have been considered. A diode laser emitting at 532nm with a power of 10 mW has been used as excitation source, whereas a mini-spectrometer connected to a laptop has been used as detector. The limit of detection, determined with eosin Y solution, has been 38 pM by employing an integration time of only 40 ms. As the typical concentration ranges considered in fluorescence labelling is often several order of magnitudes higher than the one explored in this work, the proposed approach appears as a valid strategy in fluorescence spectroscopy. For instance an optimum concentration of eosin B for protein estimation was worked out to be 0.01% in [4], whereas more limited quantity of eosin Y (10-7 mol/l) are necessary in an eosin Y-based fluorescent sensor which is used for detection of perfluorooctane sulfonate [5]. Due to the versatility of the design, the system is appropriate also for Raman spectroscopy. In this case, probe excitation configuration, exploiting the microjet waveguide capability also for the excitation represents a valid strategy due to the well-known weak sensitivity offered by Raman spectroscopy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
