Keywords

1 Introduction

Ammonia (NH3) is a nitrogen chemical compound naturally present in ground and surface water due to biological or organic decomposition processes, as well as released in ambient by industrial waste processes or farms with uncontrolled concentrations. As a direct source of nitrogen, ammonia represents one of the most popular nutrients for plants and water ecosystems although, it is also one of the most common water pollutants due to its high level of toxicity such that it is used as an important indicator of its quality and, more general, of the health of the global marine ecosystem [1]. Therefore, the monitoring of ammonia in water, even in low concentration, arouses ever increasing interest in scientific community and in the organizations responsible for the environmental protection. In this work we investigate the feasibility of an in liquid ammonia detection system based on a quartz crystal microbalance (QCM) and an oscillator circuit. The quartz surfaces, embedding gold electrodes, have been properly functionalized by means of a Self-Assembled Monolayer (SAM) of 3-mercaptopropionic acid (3MPA) solution aiming to exploits the energies attraction between the deprotonated carboxylic groups (COO−) and the NH4+ ions for the detection of ammonia traces in water. Moreover, the developed measurement setup has been conceived to overcome the problems related to the use of QCM in in-liquid applications for mass measurements [2].

2 Sensing Material and Principle

The formation of self-assembled monolayer on gold surfaces is among the most commonly used functionalization methods in QCM-based sensing applications [3, 4]. Basically, the aforementioned reaction involves the formation of a covalent bond between sulfur (S) and gold (Au) molecules followed by a self-organization step driven by electrostatic interactions and weak forces [5, 6]. The coverage, the reproducibility, and the effective formation of SAMs are influenced by many key factors, among which, the concentration of the thiol derivative solution and the cleanliness of the gold surface to functionalize [7]. The use of thiol compounds (RSH) aims to tune the specificity of QCM measurements since the SAM could provide different functionalization surfaces that may react with various kinds of analytes. Moreover, it has to be noted that the time required to the formation of the monolayer, strongly depends on the hydrophobic interactions between the involved carbon chains. Indeed, the formation of the covalent bond between Au and S happens in the first minutes of the reaction according to an exothermic process, described as follows:

$$ RSH + Au \to RSAu + e^{ - } + \, H^{ + } $$
(1)

Meanwhile, the organization into an ordered monolayer is a process that relies only on weak forces and so it requires longer times to be completed [8].

In this work, the gold QCM electrodes were chemically cleaned and successively functionalized with self-assembled monolayers of 3-mercaptopropionic acid, aiming at realizing an ammonia detection system by exploiting the electrostatic attraction between the deprotonated carboxylic groups (COO−) and the NH4+ ions in aqueous solutions (Fig. 1b).

Fig. 1.
figure 1

a) 3-Mercaptopropionic acid structure; b) interaction between ammonium ions and 3-mercaptopropionate in aqueous solution

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3 Materials and Methods

3.1 Materials

The chemical cleaning and functionalization of the QCM was realized using: Sulfuric Acid (H2SO4, 95–97%), Hydrogen Peroxide Solution (H2O2, 30% v/v), Ethanol (absolute) and 3-Mercaptopropionic acid ≥99% (3MPA), purchased from Sigma-Aldrich (Milano, IT).

The quartz used for tests is an AT-cut quartz crystal with a nominal resonance frequency fr of approximately 10 MHz and gold electrodes with 11.5 mm diameter.

Quartz impedance spectra were measured using an impedance analyzer by Wayner Kerr (6500B).

QCM measurements in transient conditions were performed by simultaneously acquiring, in real time (1 s sampling time) the quartz resonance frequency and its motional resistance exploiting a measurement system based on a Mecham oscillator circuit performing an automatic adaptive strategy for gain adjustment [2]. This feature allows to maintain the oscillator frequency close to the series resonance frequency of the quartz to monitor the physical parameters of interest, involving the quartz surface.

3.2 Methods

QCM Surface Cleaning.

To obtain a clean gold electrodes surface, the QCM were immersed in a hot piranha solution (3:1 v/v H2SO4 97% /H2O2 30%) for 5 min and then rinsed with ultrapure water. Crystals were dried under nitrogen gas flow and stored under vacuum until the next step was performed.

Self-assembled Monolayer Formation.

Clean and dry QCM crystals were immersed in a 1 mM ethanolic solution of 3MPA for 12 h to allow the chemical absorption and formation of the SAM on the gold electrodes. The functionalized crystals were rinsed with ethanol to remove the excess thiol compounds on the surface and then stored in a vacuum box.

Time of Flight Secondary Ions Mass Spectrometry (ToF-SIMS).

ToF-SIMS experiments were conducted on a TRIFT III spectrometer (Physical Electronics, Chanhassen, MN, USA) equipped with a 22 keV Au+ liquid metal primary ion source with a beam current of 600 pA and a 45° incident angle. Clean (pristine) and functionalized (3MPA) samples were analyzed after storage in a vacuum box. Positive and negative ion spectra were acquired with a pulsed, bunched primary ion beam by rastering over 100 × 100 µm and maintaining the primary ion dose below 1012 ions/cm2. Negative polarization spectra were recorded and calibrated with ions [CH]− (m/z 13.0078) [O]− (m/z 15.995) [C2H]− (m/z 25.007) [S]−(m/z 31.972) [Au]− (m/z 196.966). The resolution (m/Δm) was 4160 at m/z 27.024.

4 Results and Discussions

4.1 Surface Cleaning and Self Assembled Monolayer Formation

The presence of a SAM consisting of 3-mercaptopropionic acid was evaluated and confirmed by Time-of-Flight Secondary Ions Mass spectrometry. In particular, the results reported in Table 1, concerning the measured ions are consistent with the functionalization of the gold surface of the electrodes. In Fig. 2, the negative ions spectra in the range m/z 200–500 for both the samples: the pristine quartz in Fig. 2a and functionalized in Fig. 2b, are reported. The comparison between the obtained spectra confirms the efficiency of the cleaning procedure described above, since the main ions present in pristine QCM spectra are all attributable to a clean gold surface. The presence of peaks at m/z 228.939 (AuS−, dev 3.58 mAMU), 425.905 (Au2S−, dev −0.5 mAMU), and 457.877 (Au2S2−, dev 5.09 mAMU) are consistent with 3-mercaptopropionic acid bound to the gold surface.

Table 1. Main fragments observed in ToF-SIMS experiments
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Fig. 2.
figure 2

Negative ToF-SIMS spectra of the pristine QCM (a) and of the 3MPA functionalized QCM (b) samples. Spectral range m/z 200–500

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4.2 QCM Measurements

Figure 2a reports the measured impedance quartz amplitude, |Z|, and phase, ϕ, spectra before and after the functionalization procedure as a proof of the 3MPA monolayer formation on the quartz surface.

Transient measurements were performed by exposing one of the functionalized quartz surfaces to liquid solutions of NH3 at different concentrations. The quartz crystal is housed in a measurement chamber suitable for in-liquid applications. At first, 150 μL of ultrapure water (UW) are injected into the measurement chamber, to homogeneously cover the quartz surface and such that the liquid can be seen as a semi-infinite layer. Subsequently amounts of 30 μl of NH3 with 30% (v/v) concentration are added to cover the range: 30 μl–150 μl.

As reported in Fig. 2b a negative resonance frequency shift (Δfr) can be observed by increasing the NH3 concentration, until a saturation is reached due to the full coverage of the sensing layer, proving the formation of a strong bond between the functionalizing layer and the NH4+ ions even at the lower concentration [9]. On the other hand, the quartz motional resistance shows an opposite trend with respect to fr, showing an effective change of the properties of the sensing layer related to the molecular bond (Fig. 3).

Fig. 3.
figure 3

a-Measured quartz impedance amplitude, |Z|, and phases, spectra before and after the functionalization procedure. b-Measured quartz resonance frequency shift and motional resistance variation in time. Measurements starts by adding 150 μL of water on the QC surface, followed by subsequent additions of 30 μL of NH3 with 30% concentration (v/v) in the range 30 μL–150 μL.

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5 Conclusions

In this work the feasibility of an ammonia detecting system in water has been presented. The sensing system is realized using QCMs which surfaces have been properly functionalized with a self-assembled monolayer of 3-mercaptopropionic for ammonia detection in water. Both the system resonance frequency and motional resistance have been acquired in real time showing appreciable results in terms of ammonia trace detection.