The inline monitoring of parameters in aqueous liquids is facing an increasing demand in many different application areas. Hydrogels with pH-induced swelling and deswelling behavior offer a means to measure pH in such liquids. Here we investigate the optical interrogation of a pH-sensitive hydrogel which can be applied in the physiological pH range. For this, a nanostructured gold substrate supporting surface plasmon oscillations is coated with a HPMA/DMAEMA/TEGDMA/EG hydrogel. The gel swells in the pH range under investigation (here 4.5 to 6.5), and the resulting refractive index changes subsequently lead to a spectral shift of the plasmon resonance of the gold nanostructure. The spectral resonance position is determined from optical transmittance spectra of the sensor substrates, and the initial results for our hydrogel reported here indicate a nearly linear dependence between the swelling state and the plasmon resonance wavelength.
Efficient and low-cost sensors for the detection of chemical parameters in
liquids (e.g., pH) are facing an ever-increasing demand in many different
application fields such as medical care, biotechnology, or industrial
process monitoring. The measurement methods currently in use are typically
based on electrical sensors, which require complex calibration (Wang et al.,
2002; Wilhelm et al., 1991). An alternative approach for the measurement of
pH is based on the use of pH-responsive hydrogels. The latter are
hydrophilic, nonsoluble, crosslinked polymers, which show an increase or
decrease of volume as response to pH changes in their environment. Thus,
interrogating the swelling–deswelling state of the hydrogel allows the pH to be deduced. The volume change can be detected, for instance, by means of
a pressure sensor (Günther et al., 2013, 2014; Wang
and Ward, 1993), which demands rather thick hydrogel sensor layers ranging
from 300 to 500
The combination of a nanostructured metal surface and a pH-sensitive hydrogel was reported previously in the literature (Mack et al., 2007; Jiang et al., 2009; Tokarev et al., 2008; Yao et al., 2010). However, it is still a challenge to use hydrogels in sensor systems with a durability of 1–2 weeks, which is a typical timescale for applications in the medical and biotechnological sector. The overall aim of this work, therefore, is a robust hydrogel-based sensor for the inline measurement of pH with an operational lifetime of more than 10 days. As the first step, we report here on initial results on the optical detection of different swelling states of our pH-sensitive hydrogel on a nanostructured gold surface. As we also envision implantable sensors for biomedical applications, a hydrogel composition is investigated, which can be used in the physiological pH range (Schulz et al., 2010). The combination of this specific hydrogel and a nanostructured plasmonic sensor substrate is studied for the first time, which provides data on the interrogation of physiologically applicable gels with optical methods. The initial results reported here show a nearly linear dependence between the swelling state and the plasmon resonance wavelength of the gold structure between pH 4.5 and 6.5.
The sensor consists of an optically active nanostructured gold substrate and a pH-sensitive hydrogel layer for the analyte-specific detection of refractive index changes. Gold nanostructures as used here are well known to support localized surface plasmon polaritons, which induce a refractive-index-sensitive optical transmittance behavior (Kreibig et al., 1995). Such structures can therefore be used as signal transducers for the hydrogels. The optical interrogation of the sensor substrate is accomplished with a simple home-built transmittance evaluation system. The substrates are placed on an optical holder and are irradiated with a LED (875 nm central wavelength) in the spectral window between 750 and 1000 nm. The light transmitted through the sensor substrate is collected with an optical fiber and directed to a spectrometer (iHR550, Horiba Jobin Yvon GmbH, Unterhaching, Germany). The entire setup and a REM micrograph of the nanostructured surface are depicted in Fig. 1.
The sensor signal originates from the superposition of the incident LED light spectrum and the extinction spectrum of the sensor substrate. From this superposition, a transmittance spectrum with a characteristic dip around 870 nm results. The dip, which represents the plasmon resonance wavelength, shifts upon refractive index changes occurring during the pH-induced swelling of the hydrogel.
After fabrication and functionalization (see Sect. 3.2 and 3.3), the
nanostructured gold sensor substrate is coated with a pH-sensitive hydrogel
layer (layer thickness around 30
The sensor substrates consist of a hexagonal 3
Change of the plasmon resonance wavelength induced by the hydrogel swelling in pH 4.5 after a pre-swelling in pH 6.5. From the fit a response time of 15 min is derived after which the swelling reached 90 % of the final steady-state value (indicated by the dashed line).
The sensor substrates fabricated according to the protocol in Sect. 3.2
are cleaned for 10 min using isopropanol, high-purity water, and ethanol
under ultrasonic conditions. After drying in a nitrogen stream, the
substrates are inserted in an O
To attain different swelling degrees, the sensor substrate was placed in a petri dish containing liquids with different pH values for 15 min. For every pH value the sensor substrate was taken out of the pH solution and a transmittance spectrum was measured with the setup described above. The superposition of the Gaussian LED excitation spectrum and the plasmonic extinction spectrum leads to a characteristic dip in the overall transmittance spectrum around 870 nm (see Fig. 2a). The spectral position of the transmittance dip represents the plasmon resonance wavelength of the gold nanostructure and depends on the refractive index of the hydrogel (Zalyubovskiy et al., 2012). This resonance wavelength was determined with a Gaussian fit for the transmittance spectra of every pH value. Figure 2 shows the transmittance spectra (a) and the resonance wavelength of these measurements for the pH values between 6.5 and 4.5 with intervals of 0.5 (b).
The spectra show a blueshift of the plasmon resonance wavelengths with decreasing pH value which corresponds to a decreasing refractive index of the hydrogel during swelling. The plotted uncertainty bars originate from the uncertainty of the pH electrode, which was used for the preparation of the pH solutions, and the standard deviations of the Gaussian fits for the resonance wavelengths. Figure 2b indicates that the refractive index of the polymer decreases according to the raising content of water in the gel, which is in accordance with our expectation. The data show that the system is suitable for the detection of different pH values in aqueous liquids ranging from 4.5 to 6.5.
In order to monitor the in situ swelling of the hydrogel and to determine the response time, we placed the sensor substrates in pH 6.5 solution for 2 h. Afterwards we observed the hydrogel swelling in pH 4.5 over 90 min. Figure 3 shows the change of the resonance wavelength induced by a swelling of the pH-sensitive hydrogel over time.
The results show an exponential swelling of the hydrogel. After 15 min,
90 % of the final steady-state value is reached, which shows an improvement
of the response time by a factor of 2 in comparison to other systems
exploiting this kind of hydrogel (Schmidt et al., 2016; Schulz et al.,
2010). This improvement is caused by using a much thinner hydrogel layer (30
The error margins in Figs. 2 and 3 are still relatively large. We attribute this behavior to insufficient adhesion of the hydrogel on the substrate surface as there is no covalent binding between the hydrogel and the surface functionalization. To investigate adhesion, the substrates were placed in an ethanol–water solution for 5 days and compared to substrates on which we omitted the (3-mercaptopropyl)trimethoxy silane layer. The samples with surface functionalization show clearly better durability (flawless adhesion in the range of days instead of minutes for bare gold). However, for some of the functionalized substrates we still observe a delamination between hydrogel and the gold layer. Hence, although the current functionalization improves the adhesion it is still not strong enough to enable a stable sensor functionality. To avoid these effects, we are currently working on alternative functionalization protocols which will allow us to conduct more detailed signal stability tests.
In this paper we report on initial results concerning the optical detection of the swelling behavior of a pH-sensitive hydrogel applicable in the physiological range. We applied a nanostructured gold surface supporting localized surface plasmons as an optical transducer for refractive index changes occurring during pH-induced swelling of the hydrogel. The combination of a transducer and the HPMA/DMAEMA/TEGDMA/EG hydrogel composition is being studied for the first time. The results show a nearly linear blueshift of the plasmon resonance wavelength of the transmittance spectrum with increasing swelling of the hydrogel upon decreasing pH values. Furthermore, we could show an improvement of the response time of the sensor (by a factor of 2) in comparison to other approaches exploiting this specific hydrogel. Future work will focus on improving the adhesion between the gold and the hydrogel layer and on producing even thinner and patterned hydrogel layers.
The data presented in this article are stored in an internal system according to the guidelines of the DFG.
The authors declare that they have no conflict of interest.
The authors thank GeSiM mbH, Großerkmannsdorf, Germany for the intense
cooperation concerning plasmonic structure fabrication and Rüdiger Rosenkranz for
the REM work. Funding from the DFG research training group 1865 and the BMBF
VIP