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Development of molecularly ımprınted polymer
based quartz crystal mıcrobalance nanosensor for
the determınatıon of tryptophan
Aytaç Gültekin , Abdulkadir Ünüvar , Gamze Karanfil , Ibrahim Yilmaz &
Rıdvan Say
To cite this article: Aytaç Gültekin , Abdulkadir Ünüvar , Gamze Karanfil , Ibrahim Yilmaz & Rıdvan Say (2020) Development of molecularly ımprınted polymer based quartz crystal mıcrobalance nanosensor for the determınatıon of tryptophan, Supramolecular Chemistry, 32:6, 383-392, DOI: 10.1080/10610278.2020.1746313
To link to this article: https://doi.org/10.1080/10610278.2020.1746313
Published online: 09 Apr 2020.
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ARTICLE
Development of molecularly ımprınted polymer based quartz crystal mıcrobalance
nanosensor for the determınatıon of tryptophan
Aytaç Gültekina, Abdulkadir Ünüvarb, Gamze Karanfila, Ibrahim Yilmaz cand Rıdvan Sayd
aFaculty of Engineering, Department of Energy Systems Engineering, Karamanoğlu Mehmetbey University, Karaman, Turkey;bAbdullah Tayyar Anatolian High School, Karaman, Turkey;cDepartment of Chemistry, Kamil Özdağ Science Faculty, Karamanoğlu Mehmetbey University, Karaman, Turkey;dBionkit Ltd., Eskisehir, Turkey
ABSTRACT
In this study, a quartz crystal microbalance (QCM) nanosensor was prepared to detect tryptophan. QCM nanosensor was prepared through the formation of tryptophan memories on the gold surface of QCM electrode using Methacryloylamidohistidine-Cu(II)-tryptophan ([MAH-Cu(II)]-tryptophan) pre-organised monomer system. The designed pre-organised monomer system was characterised by use of Fourier Transform Infrared (FTIR) and Atomic Force Microscope (AFM) was used to characterise the QCM nanosensors. After the characterisation studies, imprinted and non-imprinted sensors were connected to QCM system to determine the binding of the target molecule, selectivity and the detection of the amount of target molecule in real samples. The results showed that the imprinted QCM nanosensor had high selectivity for tryptophan.
ARTICLE HISTORY
Received 11 December 2019 Accepted 12 March 2020
KEYWORDS
Tryptophan; quartz crystal microbalance; molecular imprinting; nanosensor
1. Introduction
Tryptophan is an essential amino acid with a wide range of physiological roles in the human body [1]. It is one of the amino acids that must be taken from outside in terms of nutrition. It is also necessary for the synthesis of amino acids that are not necessary for the body. Tryptophan is the need for the production of many important substances in the body, including neurotrans-mitter serotonin (5-hydroxytryptamine). It also plays an important role in biological processes, for example, pro-tein synthesis, animal growth and plant growth [2–5].
Lack of tryptophan causes the accumulation of toxic products that cause hallucination, delusion and schizo-phrenia in the brain [6]. Tryptophan overdose creates drowsiness, nausea, dizziness and loss of appetite. The level of tryptophan in the blood is related to the level of serotonin and melatonin in the brain. Therefore, trypto-phan is very important for people with anxiety. It is also used as a sleep aid, nutraceutical and antidepressant [7]. Tryptophan cannot be synthesised by the human body [8]. Therefore, this amino acid must be taken into the body through nutrition. Since tryptophan helps in the
CONTACTAytaç Gültekin aysari@yahoo.com Faculty of Engineering, Department of Energy Systems Engineering, Karamanoğlu Mehmetbey University, Karaman 70200, Turkey
SUPRAMOLECULAR CHEMISTRY 2020, VOL. 32, NO. 6, 383–392
https://doi.org/10.1080/10610278.2020.1746313
production of serotonin and melatonin, it is vital that the body obtains sufficient amounts of tryptophan-containing foods or tryptophan supplements. According to the World Health Organization (WHO), the requirement for tryptophan is 4 mg per kilogram of body weight per day. Therefore, it is important to develop a simple, fast, inexpensive and accurate method for the determination and quantification of tryptophan in different foods.
Today, many methods such as high performance liquid chromatography (HPLC) [9,10],fluorescent detec-tor HPLC [11], liquid chromatography-tandem mass spectrometry [12], spectrophotometry [13], spectro-fluorimetry [14], amperometry [15], capillary electro-phoresis technique [16], colorimetric method [17] and infrared optical sensor [18] are used to determine the amount of tryptophan. Although these methods are very important for quantitative analysis, most of them have disadvantages such as high costs, complex analysis pro-cess, long analysis time, long optimisation conditions and variables that depend on the analyst’s ability.
Quartz crystal microbalance (QCM) is a technique that can convert mass differences into an electrical signal. This technique has high frequency and is highly sensitive to mass changes occurring on the surface. The frequency change caused by the mass accumulation on the elec-trode is obtained from the Sauerbrey equation [19]. The Sauerbrey equation is as follows:
Δf ¼ fc f0 ¼
2f2 0ΔM
A ffiffiffiffiffiffiffiffiffiffippqμq
(1)
where fcis the frequency of oscillation with quartz crystal
and surface film, f0represents quartz crystal resonance
frequency, ΔM denotes added mass, pq is 2.648 g/cm3
quartz crystal density, µq represents 2.947 × 1011g/cm.
s2shear modulus coefficient for AT cut quartz crystal, Δf (fc
-f0) denotes frequency change and A is quartz crystal
surface area [20]. QCM has been widely used in biochem-istry, environment, food and clinical applications in recent years due to its superior features such as simple, inexpen-sive, efficient, space-saving and high-speed mass change. Since the QCM does not have selectivity for any chemical structure, the quartz crystal surface is modified with var-ious chemicals and bio-molecules to achieve selectivity based on a molecule. One of the most effective techni-ques used to obtain a selective polymer layer on the surface of QCM is to synthesise polymers by molecular imprinting technique [21–24].
Molecular imprinting is a useful technique that has molecular detection and recognition sites formed in a highly cross-linked polymer matrix by polymerisation in the presence of the template molecule with specific
recognition sites. Molecularly imprinted polymers (MIPs) are prepared by copolymerising excess crosslinkers in the presence of a functional monomer, a template mole-cule and a solvent. At the same time, the molecular imprinting technique is based on the process of synthe-sising polymers containing spaces on which the target molecule is recognised. These spaces recognise the size and shape of the target molecule. It removes the target molecule from the structure, resulting in functional monomers atfixed positions. Thanks to this event, con-jugate structure to the target molecule occurs. The poly-mers formed as a result of these processes recognised the structure, size and physicochemical properties of the target molecule [25]. MIPs have a unique selectivity for the target molecule and are resistant to mechanical action, heat, acid, base, water and organic solvents [26]. The methods used for the preparation of these polymers are simple and inexpensive. MIPs can be stored for several years without any change in their recognition performance. Thanks to these properties, MIPs can be used in a wide range offields such as life, pharmaceutical and natural sciences. In order to detect highly dilute samples such as nanomolar, MIPs can be synthesised at the nanometer thickness on the surface of the QCM sensors.
In our previous works, we used different metal-chelating monomers via metal coordination–chelation interactions with different molecules such as folic acid, cholic acid and caffeic acid and applied in selective sensing detection. In all of these studies we obtained short response time, wide linear range, low determination limit, high selectivity and suitability to real samples [21,25,27]. Herein we report that designing and synthesising a nanosensor based on MIPs in QCM using a metal-chelate monomer, namely, 2-metha-cryloamidohistidine-Cu (II) [MAH-Cu (II)] to determine the amount of tryptophan in different foods. It is aimed with this study to constitute a simple, economic, rapid method with less analysis procedures, less space and reusability. In addition, the developed MIP-based QCM nanosensor in this study will provide the opportunity to create a new and useful method for detecting and quantifying tryptophan.
2. Experimental studies
It is possible to collect the experimental studies under the following headings:
(1) Synthesis and characterisation of 2-methacryloa-midohistidine (MAH) and 2-methacryloamidohis-tidine-Cu (II) [MAH-Cu (II)] monomers
(2) Preparation of 2-methacryloamidohistidine-Cu (II) [MAH-Cu (II)]-Tryptophan pre-organised monomer
system and generation of tryptophan recognising memories on QCM electrode surface
(3) Use of QCM nanosensors prepared using MAH-Cu (II) monomers for tryptophan determination, stu-dies on selectivity and quantification in different food samples
2.1. Chemicals
Tryptophan, acetic acid, ethanol were obtained from Merck (Darmstadt, Germany), azobisisobutyronitrile (AIBN), and L-phenyl alanine was obtained from Aldrich (Milwaukee, WI, USA) and ethyleneglycoltimethacrylate (EDMA) was obtained from Fluka AG (Buchs, Switzerland). Ethyleneglycoldimethacrylate was distilled off in vacuo with a hydroquinone inhibitor and used after storage at 4°C.
2.2. Equipments
AT-cut quartz crystals and quartz crystal analyser (SRS Standford Research Systems, Model QCM200 Quartz Crystal Microbalance Digital Controller) were used for microgravimetric measurements. AFM images were taken with hpAFM (Nanomagnetics Instruments, Oxford, UK). Infrared spectra were taken by FTIR (Spectrum 100, Perkin Elmer, USA).
2.3. Synthesis of 2-Methacrylamidohistidine (MAH) monomer
The synthesis of the MAH monomer was carried out according to the following:
An amount of 5.0 g of L-histidine and 0.2 g of NaNO2
were dissolved in 30 mL of K2CO3(5%, v/v). The resulting
solution was cooled to 0°C. Then, 4.0 mL of metha-croylchloride was slowly added to this solution. The resulting solution was stirred at room temperature for 120 min under a nitrogen atmosphere with the aid of a magnetic stirrer. As a result of the reaction, the pH of the solution was adjusted to 7.0 and extracted with ethy-lacetate. The aqueous phase was removed using a rotary evaporator and the residue (MAH) was purified by crystal-lisation from the ethanol/ethylacetate mixture [28].
2.4. Synthesis of methacrylamidohistidine-Cu (II) [MAH-Cu (II)] metal-chelate monomer
For the synthesis of methacrylamidohistidine-Cu (II) [MAH-Cu (II)] metal-chelate monomer, 1 mmol of MAH monomer was dissolved in 15 mL of ethanol. 1 mmol CuSO4.5H2O was added slowly and continuously with
stirring. This solution was stirred for 3 h and the resulting clear blue complex was crystallised from the ethanol/ acetonitrile mixture by removal of solvent on the rotary evaporator [29].
2.5. Preparation of pre-organisation solutions of methacrylamidohistidine-Cu (II)-tryptophan [MAH-Cu (II)-tryptophan] metal-chelate monomers For the synthesis of MAH-Cu (II)-tryptophan me tal-chelate monomer; ethanol solutions containing 0.01 mmol MAH-Cu (II) and 0.01 mmol tryptophan were prepared. These solutions were mixed and stirred at room temperature for 24 h.
2.6. Tryptophan imprinted QCM electrode preparation
The gold electrode was washed with piranha solution (H2SO4:H2O2, 3:1) before polymerisation. In this solution,
the electrode was allowed to stand for 10 min, washed first with ethanol and then with distilled water and dried. The ethanol/water (4:1, v/v) solution containing 2 pro-pene-1-thiol (allyl mercaptan; SH-CH2-CH = CH2) was
dropped on the washed gold electrode surface for 1 day. Ethanol and distilled water were used to remove excess thiol groups formed in the electrode. A mixture of MAH-Cu (II)-tryptophan monomer (50 µL), cross-linker ethyleneglycoldimethacrylate (250 µL) and AIBN was dissolved in ethanol and the solution was dropped onto the quartz crystal microbalance electrode. The elec-trode surface was polymerised with ultraviolet light at room temperature for 4 h. Electrode, ethanol/acetic acid (9:1, v/v) elue solution was kept for 1 day and template removal was provided [30]. Nonimprinting nanosensor synthesised in the same way. The formation of trypto-phan memory on the QCM nanosensor surface is shown inFigure 1.
2.7. Sensor measurements with tryptophan imprinted QCM electrode
After the tryptophan removal, electrode washing dis-tilled water and drying, the constant resonance fre-quency (F0) was measured. Subsequently, the standard solution of tryptophan (16 µL) prepared in phosphate buffer was dropped onto the surface of the quartz crystal microbalance electrode and allowed to stand for 1 day. The sensor frequency was monitored until it was stable (F1). The frequency shift value for tryptophan different concentrations was calculated using the equation ΔF = F0-F1. After each measurement, tryptophan was removed by washing with an elution solution. The SUPRAMOLECULAR CHEMISTRY 385
frequency value of the sensor had been recovered up to approximately F0.
2.8. Selectivity studies with tryptophan imprinted QCM electrode
In selectivity studies with tryptophan printed QCM elec-trodes was preferred L-phenyl alanine because of its similar structure (Figure 2). The tryptophan imprinted nanosensor was first cleaned with distilled water and dried and F0 was measured. Then, a 100 μM phenyl alanine solution of (16 µL) prepared in ethanol was dropped on the surface of the QCM electrode and allowed to stand for 1 day. The frequency of the sensor was monitored until constant (F1). The frequency shift value for L-phenyl alanine was calculated using the equationΔF = F0-F1 and was then removed by washing
with elue solution. The frequency of the sensor has been recovered to approximately F0.
2.9. Quantification of tryptophan in real samples
with tryptophan imprinted QCM electrode
Milk, banana, peanut and sesame samples were used for real sample analysis studies with tryptophan imprinted QCM electrode. The acid solution was used to isolate tryptophan from bananas, peanuts and sesame samples. Filtration method was used for milk [31]. The tryptophan imprinted crystals werefirst washed with distilled water and after drying the constant resonance frequency (F0) was calculated. Banana, peanut and sesame solutions obtained from the samples precipitated in centrifuga-tion and milk sample (16 µL) were dropped on the sur-face of QCM electrode, respectively, and left for 1 day.
Figure 1.Tryptophan memory formation on QCM nanosensor surface.
The frequency of the sensor was monitored until it became stable (F1). Frequency shifts were calculated from ΔF = F0-F1 equation. After each measurement, the electrode was washed with elue solution to remove tryptophan. The frequency of the sensor has been recov-ered up to approximately F0. The amount of tryptophan in milk, and food samples were determined by using calibration studies.
3. Results and discussion
3.1. Surface characterisation of tryptophan imprinted QCM electrode
Changes in surface morphology were determined by atomic force microscopy (AFM). The AFM image of the pure gold electrode is shown inFigure 3(a). The image of the allyl mercaptan modified gold electrode is shown in Figure 3(b). The structure of the surface after polymer-isation is shown inFigure 3(c). Surface modification and polymerisation changed surface morphology. As a result of modification and polymerisation on the gold elec-trode surface, the thickness of the sensor is 322.98 nm, indicating that this is a nanosensor.
3.2. Measurement of binding interactions of tryptophan imprinted QCM nanosensors through ligand interaction
A new method based on methacroyl has been devel-oped for the preparation of tryptophan ligand-exchange polymers on gold quartz crystals and the tryptophan selectivity of MAH-Cu
(II)-tryptophan polymer has been investigated. Binding of the template molecule to the methacroyl-based metal-chelate copolymer causes a change in mass (Δm) reflected in the frequency of the crystal. The relationship betweenΔm and frequency shift was given by Saurbey equation (Equation (1)).
The tryptophan imprinted QCM electrode was washed with deionised water and then dried. The frequency (F0) was monitored and then tryptophan
solution was dripped on a confined type detector cell. With the addition of tryptophan solution, the frequency of the sensor began to decrease and reached a fixed value within 20 min (Figure 4). It was observed that the ligand-exchange reaction reached equilibrium quite rapidly. The interaction of tryptophan to the imprinted polymer on the quartz crystal is understood from the frequency change. Furthermore, tryptophan binding to the non-imprinted polymer appears to be much weaker. The binding interaction and equilibrium informa-tion between tryptophan and imprinted polymer can be obtained by the Langmuir adsorption isotherm. This analysis can be acquired using the following equation: 1 Q¼ 1 Qmax:b ½ : 1 C þ ½Qmax1 (2) where Q is the amount of tryptophan bound to polymer, as calculated by the mass frequency alteration upon addition of analyte, and C is the free tryptophan con-centration. Qmaxrefers to the apparent maximum
num-ber of binding sites, and b is the Langmuir constant. Figure 5 shows the Langmuir isotherm of trypto-phan solution in the range of 0.01–1000 µM. The results obtained from linearised the form of the Langmuir isotherm by plotting 1/Q as a function of 1/C is: 1 Q¼ 0; 0014 C þ 0; 0002 (3)
Langmuir binding constant for tryptophan binding (b) 142,857 M−1 and the number of ligand-exchange regions, Qmax5000 nmol, was calculated from Equation
3 to the MIP sensor. InFigure 5, the R2value (0.9659) is quite high. This high value indicates that Langmuir
a) b) c)
Figure 3.AFM images of (a) pure and (b) allyl modified, (c) MIP coated Scanning mode: dynamic; scanning area: 5.0 × 5.0 μm.
adsorption isotherm can be used for the tryptophan imprinted QCM nanosensor. On the other hand, the high Langmuir binding constant value indicates that the affinity in the binding regions is quite strong.
3.3. Selectivity of tryptophan imprinted QCM electrode
The result of this interaction is shown in Figure 6 and Table 1. As seen from the results, it was determined tryptophan selectivity of nanosensor was found to be nine times more according to the phenyl alanine.
3.4. Analytical performance of tryptophan imprinted QCM electrode
Calibration graph resulting frequency shift values of sensor versus tryptophan concentrations is shown in
Figure 7. A linear graph was obtained in the concentra-tion range of tryptophan 0.01 to 1000μM and. It is seen that tryptophan concentration and frequency shift values are directly proportional. The R2value of the line was found to be 0.9895. Experiments were performed in three replications. The confidence level was maintained at 95%. The limit of detection of tryptophan imprinted quartz crystal was 8.3 nM. Literature comparison of ana-lytical performances of tryptophan imprinted QCM nanosensor is given inTable 2in detail.
3.5. Determination of tryptophan in real samples with tryptophan imprinted QCM electrode
For the tryptophan analysis in real samples prepared as described in Section 2.9. The concentration of trypto-phan of milk, banana, sesame and peanut samples was determined with our sensor. The results are shown in -1800 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0 0 5 10 15 20 25 30 35 40 Frequency Shift (Hz) Time (Minute) Tryptophan imprinted nanosensor Non-imprinted nanosensor
Figure 4.QCM responses of the tryptophan imprinted and non-imprinted nanosensors.
y = 0.0014x + 0.0002 R² = 0.9659 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0.001 0.01 0.1 1 10 100 1/Q (nmol -1) 1/C (μM-1) Figure 5.Langmuir isotherm of tryptophan imprinted polymer.
Table 3. The concentrations were found by looking at the calibration graph given in Figure 7 and compared with literature (Table 3).
4. Conclusion
In this study, molecular imprinted sensor was prepared to detect tryptophan at nanomolar levels on QCM electrode surface. The binding interaction and equilibrium informa-tion between tryptophan and imprinted polymer were
obtained by the Langmuir analysis. As a result of QCM measurements with tryptophan imprinted polymer, Langmuir binding constant (b) 1.42 × 105 M−1 and the number of ligand-exchange sites (Qmax) 5000 nmol were
calculated. The high Langmuir binding constant value indi-cates that the affinity in the binding regions is quite strong. This nanosensor was used to determine the amount of tryptophan in different food samples. Results were found to support each other in the literature. By comparing the results, it was concluded that the MIP-based QCM
y = 906.03x - 175.6 R² = 0.9895 0 1000 2000 3000 4000 5000 6000 7000 0.01 0.1 1 10 100 1000 Frequency Shift (Hz ) Concentration of tryptophan (µM) Tryptophan imprinted nanosensor Non-imprinted nanosensor Linear (Tryptophan imprinted nanosensor)
Figure 7.Calibration graph for tryptophan imprinted nanosensor.
-5000 -4500 -4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 0 5 10 15 20 25 30 35 40 45 50 Frequency Shift (Hz) Time (Minute) Tryptophan phenyl alanine
Figure 6.Frequency values of tryptophan imprinted nanosensor for tryptophan and phenyl alanine (for 100 µM). Table 1.Selectivity of tryptophan imprinted polymer.
Q (mg/g) Q (mg/g) k k k’
(imprinted) (non-imprinted) (imprinted) (non-imprinted)
Tryptophan 62.86 4.87 - -
-Phenyl alanine 6.92 3.66 9.08 1.33 6.8
nanosensor was successfully prepared and suitable for real sample analysis. This nanosensor has many advantages like low cost, short response time (20 min), less analysis proce-dures, less space and reusability, wide linear range (0.01 –-1000μM), low determination limit (8.3 nM), high selectivity (9 times) and suitability to real samples. In addition, this study will provide the opportunity to create a new and useful method for detecting and quantifying tryptophan in food and health laboratories.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Ibrahim Yilmaz http://orcid.org/0000-0002-9447-3065
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