Sensor particles

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Sensor particles serve - like other sensorsalso - the transmission of a signal coming from outside into a signal form that can be processed within the system. In general, sensor particles convert external signals into a signal form that can then be transmitted without contact. Due to their small size, particle sensors can be handled like a granular material and e.g. B. can also be dispersed in a liquid. A typical example of particle sensors are optically readable chemical or biochemical sensors that locally detect the presence or concentration of a substance. In response to contact with the substance to be measured, they change their optical properties, which can then be read out as a secondary signal by an external physical measuring system.

Compared to dissolved chemically sensitive dyes or other molecular indicators that react to chemicals or environmental conditions, particle sensors have the important advantage that the sensitive elements and structures are not free but are bound to a surface or in a matrix. This makes them less susceptible to chemical and biological attack. Toxic effects such as B. occur frequently with dissolved dyes, can largely be excluded. This makes sensor particles particularly interesting for measurements in biotechnical systems . Another advantage is the variability in size of the sensor particles. In principle, such sensors can be manufactured in all size ranges down to the nanometer scale. As a result, such sensors can also be introduced into microfluidic systems. Particularly small sensor particles can also be smuggled into biological cells as “spies” and then deliver information about the physiological state or metabolic activity from inside the cell .

Fluorescent dye -based sensor particles

Sensor particles whose signal conversion is based on a change in the fluorescence intensity are particularly attractive because this signal change can be read out particularly precisely and even with very small particles. Optically readable sensor particles that indicate changes in the pH value due to a change in fluorescence ("pH sensors") use e.g. B. pH indicator dyes that are immobilized in a permeable polymer matrix. For oxygen sensor particles, dyes are used in which the quantum yield of the luminescence is dependent on the concentration or the partial pressure of molecular oxygen (O 2 ). Phosphorescence- active dyes, so-called triplet dyes, serve as indicator molecules. The signal arises due to the extinction of the phosphorescence by the triplet character of the molecular oxygen O2. The optical signal obtained is therefore more intense, the lower the oxygen content in the vicinity of the particle.

SERS sensor particles

While a change in a color or the fluorescence intensity means only moderate selectivity in signal generation, much more specific material information can be obtained from vibrational spectroscopic data. However, these are difficult to access in classic infrared sensors , especially in aqueous systems. Instead, with surface- enhanced Raman spectroscopy (SERS), very good vibration spectroscopic data can also be obtained from aqueous systems.In SERS sensors, the reinforcement effect of metal surfaces - especially on silver - is used to measure even with small measurement volumes and relatively low concentrations (millimolar and below ) to gain high signals of Raman scattering . It can be excited with visible light or also with NIR or UV light , so that there is generally sufficient transparency even in aqueous solutions. The prerequisite is that the metallic component is finely distributed. The direct application of metal nanoparticles is often problematic, both because of the handling and because of undesired adsorption effects or biological interactions. Composite sensor particles offer a very attractive alternative here: These particles typically have dimensions in the micrometer to sub-millimeter range. They consist of a swellable polymer matrix which, when swollen, is permeable to the analyte molecules. The actually Raman-reinforcing metals are embedded in this matrix in the form of nanoparticles. In this way, high sensitivity and easy handling of the sensor particles can be combined.

Application of sensor particles in microfluidics

In biological screenings in which multiphase systems, droplets or microfluid segments with volumes from the lower microliter to the picoliter range are used, the use of conventional chemical or biochemical sensors is difficult. In contrast, small sensor particles can be introduced into the fluid compartments and also transported with the fluids. Therefore, they are ideally suited for the determination of material parameters in such microfluidic environments.

Examples of such applications are for the monitoring of pH and oxygen in miniaturized biotechnology, for example in the cultivation of bacteria in microfluid segments. A change in the material composition recorded via the particles - e.g. B. the pH value or the oxygen partial pressure - can also be used as an indicator for a reduction in metabolic activity due to toxic effects in microtoxicological studies.

Use of microfluidics for the production of sensor particles

In order to produce sensor particles with the same properties, the size, shape and composition of the particles must be as homogeneous as possible. Since the particles are often produced in a relatively broad distribution of their properties in conventional manufacturing processes, microfluidic techniques are now being developed with which very high particle homogeneities can be achieved. This not only applies to simple and composite polymer particles, but above all to the shape-anisotropic (non-spherical) particles and composite microparticles that are particularly interesting for particle sensors.

Individual evidence

  1. ↑ J. Cao et al .: Data transfer from fluidic microcompartments: micro- and nanoparticles as optochemical primary transducers in miniaturized biotechnology (proceedings, 15th Heiligenstädter Colloquium, 17-29 September 2010), 273-284
  2. ↑ S. Nagl, MIJ down, M. Schäferling, OS Wolfbeis: Method for simultaneous luminescence sensing of two species using optical probes of different decay time, and its application to to enzymatic reaction at varying temperature, Anal. Bioanal. Chem. 393: 1199-1207 (2009)
  3. ↑ G. Cristobal et al .: On-line laser Raman spectroscopic probing of droplets engineered in microfluidic devices, Lab Chip 6 (2006), 1140-1146
  4. ↑ K. Strehle et al .: A reproducible surface-enhanced Raman spectroscopy approach. Online SERS measurements in a segmented microfluidic system, Anal. Chem. 79: 1542-1547 (2007)
  5. ↑ JM Köhler et al .: Polyacrylamid / silver composite particles produced via microfluidic photopolymerization for single particle-based SERS microsensorics, Anal. Chem. 85 (2013), 313-318
  6. ↑ N. Visaveliya et al .: Composite sensor particles for tuned SERS sensing: microfluidic synthesis, properties and applications, Appl. Mater. Interfaces 7 (2015), 10742-10754
  7. ↑ A. Funfak et al .: Monitoring cell cultivation in microfluidic segments by optical pH-sensing with a micro flow-through fluorometer using dye-doped polymer particles, Microchimica Acta 164 (2009), 279-286
  8. ↑ J. Cao et al .: Oxygen sensor nanoparticles for monitoring bacterial growth and characterization of dose / response functions in microfluidic screenings, Microchimica Acta 182 (2015), 385
  9. ↑ Ch. A. Serra et al .: Engineering polymer microparticles by droplet microfluidics, J. Flow Chem. 3 (2013), 66-75
  10. ↑ A. Knauer, JM Köhler: Screening of nanoparticle properties in microfluidic syntheses, Nanotechnol. Rev. 3 (2015), 5-26
  11. ↑ N. Visaveliya et al .: Composite sensor particles for tuned SERS sensing: microfluidic synthesis, properties and applications, Appl. Mater. Interfaces 7 (2015), 10742-10754