Biosensors for Detection of Ochratoxin A
Mycotoxins such as ochratoxin A, aflatoxin B and others are dangerous food contaminants that usually occur in trace amounts from nanograms to micrograms per gram of food. Therefore high sensitive methods are necessary for their detection. The conventional methods such are high-performance liquid chromatography (HPLC), mass spectroscopy are rather expensive and time consuming, therefore biosensor technology is rather promising for rapid detection of toxicants in field conditions, far from specialized laboratories. Among biosensors based on affinity of monoclonal antibodies or DNA aptamers to mycotoxins are of special interest, because provide sensitivity of detection that is better than allowable quantities of toxicants in food. While antibodies are traditional receptors in biosensors, aptamers are novel biopolymers with the affinity comparable to that of antibodies. However in contrast with antibodies, aptamers are more stable and the biosensors based on DNA aptamers can be regenerated which allowing their multiple use. This contribution reviews recent achievements in development affinity biosensors for detection ochratoxin A.
KeywordsMycotoxin Ochratoxin A Antibodies DNA aptamers Biosensors
The natural toxins, such are mycotoxins, abrin, ricin, saxitoxin, palytoxin, batrachotoxin, botulinum neurotoxin type A, mycrocystin–RC represent considerable hazard for health and could be considered as potential warfare agents . Among mycotoxins the ochratoxin A (OTA) is of special interest. OTA belongs to toxical fungal metabolites that can occur in primary food products. OTA is produced by Aspergillus ochraceus and Penicillium verrucosum and generally appears during improperly storage of cereals, coffee, cocoa, dried fruit, pork etc. and occasionally in the field of grapes. It may also be present in blood and kidneys of animals that have been fed on contaminated feeds. In a blood OTA is bound to the serum proteins and is redistributed to various tissues. The most susceptible to OTA are kidneys. It has been shown, that accumulation of OTA in this organ causes acute and chronic lesions by affecting the anion transport . This molecule causes also other toxicological effects including hepatotoxic, neurotoxic, teratogenic, immunotoxic. The toxicity of OTA is in particularly connected with inhibition of protein synthesis because it competes with phenylalanine in the cells that utilizing this amino acid. Animal studies indicated that OTA is carcinogenic . This is connected with the genotoxicity of OTA because it induces oxidative stress in the cells [2, 4], oxidative base damage  and the cleavage of single stranded DNA [6, 7]. OTA interacts also with double stranded DNA (dsDNA), but the damage of double helix has not been observed . The mechanism of OTA-induced cancer is not clear yet. Moreover the contradiction results were reported in OTA behavior in vivo and in vitro conditions. While in vivo OTA is poorly metabolized , in vitro investigations suggest that a hydroquinone/quinone redox couple and a carbon-bonded OTA-deoxyguanosine adducts are formed by electrochemical oxidation and photoreaction of OTA which may be the reason of OTA carcinogenicity .
Due to high toxicity of OTA, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) already in 1991 evaluated a provisional tolerable weekly intake (PTWI) of 112 ng/kg body weight (b.w.) for this mycotoxin. This evaluation was based on the porcine nephropathies data . Most recently, the European Commission has fixed maximum concentration of OTA in foodstuffs: 3 μg/kg (7.4 nM) for cereal products, 5 μg/kg (12.4 nM) for roasted coffee and up to 10 μg/kg (25 nM) for instant coffee. Similar contamination limit was fixed for dry grapes (10 μg/kg) (EC No. 466/2001, 1881/2006), but 2 μg/L (5 nM) contamination limit is valid for wine (EC No. 123/2005). Even stronger limit of contamination by OTA was fixed for all food preparation for babies (0.5 μg/kg, EC No. 466/2001). The cereals, especially in countries with hot climate are contaminated by OTA produced by Aspergillus ochraceus. But in countries with colder climate the contamination is due to Penicillium verrucosum. The studies performed by Pittet  suggest that 25–40% of cereals are contamined by mycotoxins worldwide. Many works reports also on contamination of grape juices with approx 7 μg/L OTA (see  and references herein).
So far 15 naturally occurred ochratoxins were identified. Most frequently appeared are ochratoxins A, B, C, from which OTA is more prevalent. The differences between ochratoxins are in some cases minor. For example replacement of Cl on H yields in ochratoxin B . At neutral pH (pH = 7) in a water OTA is negatively charged. This is due to ionization of carboxyl (pKa = 4.2–4.4) and phenolic moieties (pKa = 7.0–7.3) [12, 13]. Therefore both mono (OTA−) and dianions of OTA (OTA2−) are present at physiological pH.
OTA contamination is typically in trace amount from ng to μg per gram of foodstuff. Therefore sensitive analytical methods of detection should be applied. At present the OTA detection is performed mostly by high-performance liquid chromatography (HPLC) with fluorescence detection (OTA posses natural fluorescence) , gas chromatography connected with mass spectrometry (GC-MS) , capillary electrophoresis , radioimmunoassay  or enzyme-linked immunosorbent assay (ELISA) [18, 19]. Association of Official Analytical Chemists (AOAC) official methods for determination of OTA in food are based on HPLC. However, traditional methods are rather expensive, time consuming and could be performed by qualified staffs only in specialized laboratories. In addition these methods usually require organic solvents for extraction toxines from food, which represent additional pollution for environment. ELISA belongs to the rapid detection techniques, however the disadvantage is in necessity of using enzyme-labeling reagents, which are expensive. Therefore, there is urgent requirement for direct, rapid, and low costs methods for OTA detection.
The biosensor technology fulfills the above-mentioned requirements. The biosensor is portable device consisting of sensing element, which has usually biological nature, for example antibody, enzymes, DNA, DNA aptamers, natural receptors. However, even the systems that mimic the biological structures, for example calixarenes incorporated into the lipid films can also be considered as belonging to these devices [20, 21]. The next part of the biosensor is transducer that transform usually chemical signal to the electrical, optical or mass. This signal is analyzed by separate instruments, for example potentiostats, network analyzers, spectrometers, surface plasmon resonance (SPR) etc. The tendency however exists in the integration of sensing element and transducer into one chip, such as field effect transistor. The principles of biosensor construction have been explained in many books and reviews (see for example [22, 23, 24]).
The label free detection of OTA can be based on its redox properties which allowing OTA detection at certain type of surfaces using electrical methods, such as cyclic, differential pulse or square wave voltammetry. However, more sensitive are the methods utilizing specific receptors, for example the enzymes, antibodies or DNA aptamers, immobilized at surfaces. This contribution reviews recent achievements in development biosensors for detection OTA and contains also results obtained in author’s laboratory.
10.2 The Biosensors for Detection OTA
10.2.1 OTA Oxidation and Its Detection by Amperometry
We already mentioned above that OTA carcinogenic effect may be connected with its oxidation, which causes appearance of reactive oxidation species in the cells. Oxidation of OTA is connected with its phenolic moiety [8, 25, 26]. It has been shown, that OTA can be oxidized at glassy carbon electrode (GCE) at specific conditions in organic solvents or aqueous media with pH between 6 and 8. Oxidation of OTA was studied also in presence of transition metal ions  and of a Fe-porphyrin system. In later case a hydroquinone species were detected by HPLC.
Detailed study of OTA oxidation at GCE at wide pH range (2–12) was performed by Oliveira et al. . Using cyclic, differential pulse and square wave voltammetry they observed well resolved redox peaks. The peaks appeared at acidic conditions (pH = 4.0) at potentials +1.05 and 0.45 V vs. Ag/AgCl reference electrode were used for analytical purposes. It has been shown that using square wave voltammetry it is possible to detect OTA with the limit of detection (LOD) 0.26 μM at optimal conditions. They also analyzed possible interferences with catechol, phenol and reveatrol, which significantly affected the OTA detection. Despite the fact that the LOD obtained by voltammetry at bare GCE is insufficient for practical applications, the method is useful for analysis of interaction of OTA with dsDNA. As mentioned above, authors observed interaction of OTA with dsDNA without causing its damage.
The electrochemical studies performed by Calcutt et al.  predicted that peroxidases could participate in OTA oxidation. This has been approved in work by Alonso-Lomillo et al. [27, 28] and used for detection OTA. Oxidation of horseradish peroxidase (HRP) at presence of hydrogen peroxide resulted in oxidation of OTA in aqueous solution. At the surface of screen printed carbon electrode (SPCE) at certain potential (around −0.3 V vs Ag/AgCl reference electrode) further oxidation of OTA took place. It is observed as an increase of anodic current in chronoamperometry experiment. It has been shown that immobilization of HRP has substantial effect on the sensor sensitivity. In Ref.  the HRP was immobilized at the surface of polypyrrol layer electropolymerized at SPCE. The OTA was in this case detected with LOD 0.25 nM (0.1 ng/mL). In most recent work the SPCE was prepared using carbon ink containing HRP . However, this resulted worse detection properties of OTA (LOD 26.8 ± 3.6 nM). On the other hand in both cases the matrix effect of beer or roasted coffee was rather small and sensor revealed recovery 103% and 99%, respectively . Rather high sensitivity in OTA detection was reported by Perrotta et al. . They used square wave voltammetry for detection of OTA at the surface of gold electrode modified by cysteamine self-assembled monolayers. The detection and quantification limit was 4 ng/L and 12 ng/L, respectively. This is rather high sensitivity, but selectivity of this assay was not investigated,. They also performed test of the sensor in a red wine with a recovery ranging from 94% to 145%. However, without specific receptors the amperometric detection of OTA can not be considered as a selective method due to various potential interferences from electroactive species that are present in large amount in a real samples, for example ascorbic acid, phenolic compounds etc. Large (145%) value of recovery in ref.  suggests that such interferences are very likely.
10.2.2 Amperometry OTA Biosensor Based on ELISA
Immunochemical methods of detection are of high sensitivity and allowing detection of OTA at concentrations below the EC regulatory values. The most popular immunochemical method is ELISA. The conventional method is based on ELISA optical microplate reader using spectrophotometric detection. The detection is performed in a small volume (around 100 μL) in a well of microplate. Microplate containing usually large number of wells (typically 96), which allowing fast and simultaneous detection of several analytes. The product of enzyme reaction that posses absorbance or fluorescence signal is detected by optical reader. The detection can be indirect or direct and is based on competitive assay .
In indirect detection format usually 100 μL solution of OTA conjugated with bovine serum albumin (BSA) is added into the well and kept at 4°C overnight. Then 100 μL of 1% polyvinilalcohol (PVA) is added to block the microwells (1 h at 37°C). The anti-OTA IgG conjugated with enzyme alkaline phosphatase (AP) is added into the well in a sample containing unknown concentration of OTA. With increased amount of OTA in a sample less number of anti-OTA IgG is bounded to the OTA-BSA. AP transforms substrate 1-naphtyl phosphate into the product 1-naphtol (NP). The product absorbs light at wavelength 405 nm and thus can be detected by colorimetry. Therefore after addition of the substrate NP is detected. The amount of NP is indirectly proportional to the OTA.
In direct ELISA approx. 100 μL solution of anti OTA IgG is added into the well and incubated overnight at 4°C. The well is then blocked by PVA like in indirect assay. The OTA-AP conjugate is then added in a sample containing unknown concentration of OTA. OTA-AP will compete with free OTA in a sample. Thus, with increased concentration of OTA, less number of OTA-AP complexes will bind to anti OTA IgG. The concentration of OTA-AP is detected by the same method like in an indirect assay.
The conventional colorimetry based ELISA has been used so far in a large number of works focused on detection various toxins, such are pesticides, marine toxins [31, 32], fungal toxins (aflatoxin, tricothecene) , OTA [34, 35, 36, 37, 38], ricin [39, 40] bacterial toxins [41, 42, 43, 44]. See also [1, 45] for review. Simultaneous enzyme assay for screening the aflatoxin and OTA was also reported . In this assay monoclonal antibodies were immobilized onto the nitrocellulose membranes. The method allowed detection of aflatoxin and OTA with LOD 2 and 10 μg/kg for aflatoxin and OTA respectively and has been successfully used for determination these mycotoxin in chili samples. High sensitive indirect ELISA in a flow format with chemiluminiscence detection was reported in Ref. . In this assay the OTA was immobilized at the glass plate using peptide linker. The LOD 0.01 μg/L (approx. 74 pM) belongs to the most sensitive reported so far. The method was useful for practical applications and has been approved for detection OTA in a roasted coffee.
The amperometric biosensors for detection OTA appeared only in 1987 when Aizawa  reported biosensor for detection OTA in ELISA like format. The sensor was composed of amperometric oxygen electrode and OTA covalently bound to a membrane that covered this electrode. The anti-OTA antibody labeled with enzyme catalase has been added in a fixed quantity to the sample solution and allowed to react competitively with the OTA immobilized at the membrane and with free OTA The catalase activity (production of oxygen) was inversely proportional to the concentration of OTA in a sample. The sensor allowed OTA detection with LOD 0.1 μg/L (0.25 nM, or 0.1 ppb).
In contrast with traditional methods such is HPLC in which the analyte to be detected is extracted from real sample using various solvents, the ELISA is different. The analyte is detected in a real sample. Therefore the matrix effect could affect the detection and should be therefore specially analyzed and detection assay optimized. The most sensitive methods detect OTA in real samples with sensitivity in μg/kg range in a batch [48, 49] or in a flow format . Even 0.05 μg/kg LOD was reported for indirect immunoassay in microfluidic format with antibodies immobilised on magnetic nanoparticles. This assay was approved for detection OTA in apples . High sensitivity in an indirect enzyme assay was reported in Ref. . In this work the OTA-ovalbumin conjugates were immobilized on a gold colloid layer. The competitive amperometric detection was performed by addition of free OTA and specific antibodies conjugated by alkaline phosphatase with detection limit 8.2 ng/L and validated in a corn samples. The appearance of screen-printed electrodes that integrate the working electrode (carbon, gold, etc.) with reference Ag/AgCl and counter electrode resulted in further advantage of immobilization of the specific receptors [53, 54]. The recognition elements can be even entrapped into the ink during preparation of the sensing layer . Optimization of immobilization and detection condition allowed detecting OTA with substantially improved sensitivity in a direct ELISA format with LOD up to 0.05 μg/L .
The enzyme based biosensors for detection OTA are rather useful, but still require enzyme conjugation and laborious preparation. Currently, only conventional ELISA kits are available at the market. But several hours are typically required for obtaining the result. Faster kits appeared recently (Veratox, Neogen, Lasing, USA) allowing detection of OTA within 30 min, but sensitivity is lower. Therefore there is attempt to develop another specific immunoassay that does not require application of antibody/enzyme conjugates. In this respect the focus is mostly on application of the surface plasmon resonance (SPR), quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS) for detection OTA at surfaces modified by specific antibodies. The overview of achievements in development of SPR, QCM and EIS based immunosensors is presented in next section.
10.2.3 Immunosensors Sensitive to OTA
In this part we will analyze the works focused on development SPR, QCM and EIS based immunosensors for detection OTA. The main advantage of these devices is lack of enzymes and in some cases possibility of direct detection of OTA.
SPR method belongs currently to the standard one for detection affinity interactions at surfaces. This is due to high sensitivity, selectivity, possibility of surface regeneration and due to existence of powerful commercial instruments; such are Biacore (Sweden) or Spreeta (Texas Instruments, USA). The principles of SPR are described in many textbooks and reviews (see for example ). SPR is based on generation of plasmons in a thin layer adjacent to the glass prism by irradiation of laser beam at certain angle of incidence. The angle of reflected beam shifts with increasing the thickness of sensing layer. Thus, measurement of this angle allowing determination of the surface concentration of adsorbed molecules as well as the thickness of the layer . This method has been used for multiple detection of mycotoxins, including OTA within 25 min with sensitivity approx. 0.5 ppb (0.5 μg/L) . SPR device combined with molecularly imprinted polymer method (MIP) allowed detection of OTA within a linear range from 0.05 to 0.5 mg/kg . Yuan et al.  proposed interesting approach. They used IgG-gold nanoparticle conjugates for amplification the SPR signal following competitive interaction of OTA and monoclonal antibodies with the surface of specifically immobilized OTA. The indirect competitive assay was based on immobilization of OTA conjugated to BSA or OTA connected to polyethylene-based linker (PEG) to a Biacore chip. Simultaneous addition of OTA and monoclonal antibody specific to OTA (mAb) resulted in SPR signal due to binding of mAb to a surface. With increased concentration of OTA, more mAb bind to free OTA in comparison with that at the surface. Thus SPR signal was indirectly proportional to the OTA concentration. Approx. ten fold better detection limit was obtained for OTA immobilized to a surface through PEG linker (1.5 μg/L) in comparison with OTA-BSA conjugate. This has been attributed to higher density of OTA at surface. The PEG linker was also important in providing less nonspecific interactions of species with the sensor surface and allowed substantially improving surface regeneration (up to 600 binding cycles) using 5 M guanidine in 50 mM glycine (pH = 2). The assay as well as surface regeneration was performed in a flow format (25 μL/min). Using gold nanoparticles of a diameter 40 nm conjugated to anti mouse IgG it was possible to amplify the SPR detection reaching LOD 0.042 ± 0.004 μg/L. The sensitivity is higher in comparison with requirements of allowable contamination of foodstufs indicated by food and environmental agencies. The method was successfully applied for detection OTA in oat, corn, white and red wine, grape and apple juice spiked samples. Despite rather high detection limit there is still disadvantage consisting in indirect assay and in necessity to use OTA-mAb conjugates.
Most recently the direct SPR biosensor for OTA detection was reported by Zamfir et al. . The monoclonal antibodies specific to OTA were immobilized at SPR chip surface using magnetic beads. The addition of OTA resulted in increase of SPR angle. The detection limit obtained (0.94 μg/L) is higher in comparison with previous indirect approach, but still sufficient for practical applications.
QCM method belongs to powerful tools for analyzing the affinity interactions at surfaces. The method is based on using specially cutting quartz crystal (AT cut) and modification of its one side by sensing layer. At certain frequency, typically between 5 and 10 MHz the shear oscillation of the crystal takes place. The resonant frequency of the oscillations is indirectly proportional to the mass of the sensing layer . It should be, however, note that in a liquid the crystal oscillations are affected by viscosity forces, which resulted in damping of the acoustic wave amplitude [61, 62]. However, in certain cases the measurement of only oscillation frequency can be useful for analytical purposes. The advantage of QCM is in much lower price of instrument in comparison with SPR and in easy operation. QCM immunosensor for detection OTA by indirect assay was reported in paper by Tsai and Hsieh . They immobilized anti OTA Ab at the surface of 16-mercaptohexadecanoic acid (16-MHDA). Then the OTA conjugated with BSA in a solution containing free OTA in different concentrations was added to a QCM surface. This competitive assay exhibited a working range of 50–1,000 μg/L and a detection limit of 16.1 μg/L. The sensor was applied to several real samples with recovery in a range of 76–142%. The detection limit is lower in comparison with SPR method. However, amplification by nanoparticles could improve this.
EIS method has been so far extensively used for detection affinity interactions in biosensors . It is based on high sensitivity of impedance to the surface modification. The sensitivity is substantially enhanced at presence of redox probe, for example [Fe(CN)6]−3/−4. At certain redox potential the probe provides enhanced electron exchange between the probe and the electrode surface. EIS allowing determination of the charge transfer resistance Rct that is measure of the intensity of charge transfer. Higher electron transfer corresponds to lower Rct and vice versa. The modification of the sensor surface resulted in changes of Rct. Addition of analyte also yields in Rct changes and may be affected by analyte charge. For example, because OTA is negatively charged at neutral pH, its adsorption to a sensor surface will result in repealing the redox probe. Thus, increased concentration of OTA will cause increase in Rct. This approach has been used in recently reported OTA immunosensors, which differ mostly in the method of immobilization of OTA specific antibodies. Chronologically, the EIS sensor based on OTA specific Ab immobilized on a surface of chitosan-polyaniline conducting polymer prepared by electropolymerization on a indium–tin-oxide (ITO) electrode was reported by Khan and Dhayal [65, 66]. The highest sensitivity of 1 μg/L was obtained, which is comparable with direct SPR assay. Shortly after these works Radi et al.  published paper in which Ab was immobilized on a gold surface using carbodiimide chemistry (LOD 0.5 μg/L). Rather high sensitivity (LOD 6 ng/L) was obtained with EIS immunosensor in which the OTA specific antibodies were immobilized on nanostructured zinc oxide deposited onto ITO covered glass plate  or using carbon nanotubes as immobilization matrix (LOD 2.5 nM/L) . Comparable LOD (0.01 μg/L) was obtained in Ref.  in which Ab were immobilized by means of magnetic beads. Rather high sensitivity of EIS biosensor based on platinum electrode with electropolymerized sulfonated polyaniline with incorporated polyclonal anti OTA antibody was reported most recently . The detection limit indicated 10 pg/kg, however seems to be overestimated considering the sensor response at the OTA concentrations studied 2–10 μg/L. The problem arises also with calibration curve presented in this paper as a plot of charge transfer resistance, Rct, vs OTA concentrations. While Nyquist plot indicates the decrease of Rct value, the calibration plot revealed opposite direction. From Nyquist plot it is also evident saturation of Rct at higher OTA concentrations, but in a corresponding concentration range the Rct value is presented as a linear plot vs, OTA content. In most works the analysis was performed also in real samples with minor matrix effect. However in certain cases the discrepancy between the concentrations of OTA in samples provided by vendor was substantially different in comparison with that determined by EIS biosensor and ELISA test. For example see  for detection OTA in roasted coffee and wheat. Thus, EIS immunosensors are rather useful and sensitive tools for detection OTA. Unfortunately in most cases the sensors were disposable without possibility of surface regeneration. Another disadvantage of immunosensors is necessity of rather laborious and expensive preparation of monoclonal antibodies and hapten conjugates. Antibodies are unstable in multiple uses. As an alternative the DNA aptamers are of high promising receptors in biosensing applications that could replace antibodies. In a next section the biosensors based on these novel biopolymers sensitive to OTA are presented.
10.2.4 Aptamer Based Biosensors for OTA Detection
Biosensors based on DNA aptamers (aptasensors) are of growing interest due to their high sensitivity and selectivity [71, 72, 73, 74, 75]. This is particularly due to unique properties of DNA or RNA aptamers – the single stranded nucleic acids with high affinity to proteins or to other low and macromolecular compounds, which is comparable with that of antibodies. In contrast with antibodies, aptamers are synthesized in vitro by the SELEX procedure [76, 77]. Aptamers are thermally stable, reusable and once selected they can be produced by chemical synthesis in necessary quantity by means of conventional oligonucleotide chemistry. Aptamers can be chemically modified by biotin, thiol or amino groups, allowing them to be immobilized on various solid supports. In contrast with antibodies aptamers are more stable and the aptasensors can also be regenerated. This opens new routes for construction of biosensors for practical applications, for example for diagnosis purpose in medicine or for detection toxicants in food or in the environment. Recently the DNA aptamer sensitive to OTA has been developed . This aptamer has the following oligonucleotide sequence: 5′ GAT CGG GTG TGG GTG GCG TAA AGG GAG CAT CGG ACA 3′. The analysis of this sequence using mfold program  shows two structures containing loops that slightly differ by Gibbs energy. Energetically more favorable structure (ΔG = −1.2 kcal/mol) contains loop between 16 and 28 nucleotides. Larger loop was found for second structure (ΔG = −0.88 kcal/mol). The analysis of the aptamer structure using the QGRS Mapper program predicting the existence of guanine quadruplexes  we showed that the OTA sensitive aptamer contains one guanine quadruplex connected by loops . The existence of quadruplex is supported also by our recent data on the study of OTA aptamers thermodynamic properties . The phase transition temperature for most stable aptamers at presence of 20 mM Ca2+ has been 48.3 ± 0.5°C, which is close to the melting temperature of DNA aptamers sensitive to fibrinogen binding site of thrombin that also contain one guanine quadruplex. For this aptamers the quadruplex structure has been well established using various methods including circular dichroism (CD) . Most recently the existence of quadruplex in OTA aptamer has been approved by CD method . The changes in Gibbs energy for OTA aptamers determined from melting data was 3.8 kcal/mol, which is higher in comparison with that obtained from mfold program. However, this program does not taking into account the quadruplexes, which are rather stable also thanks to stabilizing role of K+ ions.
The binding site for OTA in the aptamers is not known yet. However, as it has been shown in original work by Cruz-Aguado and Penner  this aptamer has rather high affinity and selectivity to OTA. The affinity substantially increases at presence of 20 mM Ca2+ (constant of dissociation KD = 49 nM). At the same time, no binding of OTA was observed without calcium or magnesium ions. However, we have recently shown that OTA sensitive aptamers modified by thiol groups and immobilized at gold surface by chemisorption can bind OTA even when no Ca2+ is present. Moreover, the constant of dissociation is lower in comparison with that in solution, which is evidence of improved affinity properties of the aptamers at the surface .
According to WOS database the first aptasensor for OTA was reported most recently and utilized electrochemiluminiscence method of detection (LOD 17 pM) . The sensitivity of chemiluminiscence method of detection OTA by aptasensor was recently substantially improved by using Fe3O4 based magnetic nanoparticles (MNPs) and upconversion nanoparticles (UCNPs) as sensitive labels . The assay was based on immobilization of aptamer DNA 1 sequence onto the surface of MNPs, which allowed capturing and concentrating OTA from bulk samples. The aptamer DNA 1 sequence then hybridized with UCNPs modified with DNA 2 sequence, which could dissociate from DNA 1 and result in a decreased luminescent signal when aptamer DNA 1 recognized and bound to OTA. Under the optimal conditions, the decreased luminescent intensity was proportional to the concentration of OTA in the range of 0.1 ng/L to 1 μg/L with a detection limit of 0.1 ng/L (0.25 pM). This method allowed measurements of OTA in naturally contaminated maize samples.
Aptasensor utilizing amperometric detection based on methylene blue (MB) as a redox probe has also been reported . In later case the effective detection range of OTA was 0.25–49.5 nM (sensitivity of detection: 74.3 pM). Such a high sensitivity has been achieved by signal amplification using gold nanoparticles. Three DNA oligonucleotides were used: 12-mer DNA 1 modified by amino group at 3′ end was immobilized to the activated surface of glassy carbon electrode (GCE). The aptamer, 36 – mer DNA 2, containing complementary part to DNA 1 was then added and allowed to hybridize with DNA 1. Finally, 12-mer DNA 3 thiolated at 5′ end containing complementary part at 3′ end and modified by gold nanoparticles at 5′ end was added and allowed to hybridize with DNA 2. Addition of MB, that selectively binds to guanine residues posses well resolved CV signal with two redox peaks at −0.23 and −0.18 V vs. saturated calomel electrode (SCE). Addition of OTA resulted in folding of the aptamers into 3D configuration and caused removing of the DNA 2 and DNA 3 from the sensor surface. Because MB was bounded mostly with DNA 2 this removal also caused decrease of the amplitude of redox peaks that served as analytical signal. The sensor selectively detected OTA in comparison with aflatoxin B. However slight interaction was observed with OTA analogue – ochratoxin B. Sensor was validated in a red wine with a good recovery in a range of 95–110%.
Further the electrochemical aptasensor based on indirect and direct competitive assay (LOD 0.27 nM) has been developed . In an indirect assay the biotin-OTA conjugates were immobiližed on a surface of magnetic beads coated by streptavidin. The magnetic beads were attached to the SPCE by magnet. The competitive assay was performed by addition of various concentration of OTA in a buffer containing fixed concentration of OTA-sensitive aptamers conjugated with HRP. The presence of HRP was detected chronoamperometrically at presence of the H2O2, substrate of HRP at potential −0.2 V vs. Ag/AgCl electrode. The amplitude of current was proportional to a surface density of HRP and indirectly proportional to the concentration of OTA in a solution. The LOD in an indirect assay (1.1 μg/L) was similar to that obtained in competitive indirect immunoassay . In a direct assay the aminated aptamers were immobilized in a magnetic beads coated by carboxylic acid using carbodiimide chemistry. The beads were attached to a surface of SPCE by magnet. Free OTA was added in a solution containing fixed concentration of AP-OTA conjugates. The surface density of AP was measured by differential pulse voltammetry (DVP) at presence of the non-electroactive substrate 1-naphtyl phosphate (1-NP). 1-NP has been dephosphorylated by AP into electroactive 1-naphtol, which was oxidized at electrode to 1-iminoquinone. Oxidation current was measured at the range 0–0.4 V vs. Ag/AgCl. Similarly like for indirect assay the amplitude of current was indirectly proportional to the concentration of OTA. The indirect assay was ten fold more sensitive in comparison with indirect one (LOD 0.11 μg/L or 0.27 nM). The sensitivity of this sensor was validated in spiked wine with a good recovery in a range 94–97%.
The current was inversely proportional to the concentration of OTA. Authors also confirmed that at presence of 20 mM Ca2+ the signal increased by approx. 12% due to improved binding of OTA to the aptamers. At presence of Ca ions negatively charged OTA probably easier binds to the negatively charged aptamers. The binding of OTA was selective. Approx. 100 fold less binding took place for ochratoxin B and structural components of OTA – L-phenylalanine and warfarin. The sensor was validated in OTA containing wheat standard with recovery ranged from 102% to 104%. The sensor revealed higher sensitivity in comparison with immunosensor utilizing similar detection method . The disadvantage of the sensor consisted in necessity of using enzyme conjugates as well as in possible non-specific interactions of conjugates with SPCE.
The direct, one step detection of OTA would be, however, more advantageous for practical applications. Most recently the simple colorimetric method of OTA detection has been reported . In this work the gold nanoparticles were modified by OTA sensitive aptamers. Addition of OTA resulted in removal of the aptamers from the surface of nanoparticles and after addition of salts the changes in color has been observed due to nanoparticle aggregation. This method allowed detection of OTA with LOD of 20 nM.
Another approach for direct detection of OTA is based on EIS electrochemical aptasensors. The results in development of such aptasensor for detection OTA utilizing thiolated OTA specific aptamers chemisorbed on a gold surface have been presented by us in AISEM conference in February 2011 . The approach is similar to those for EIS immunosensor at presence of redox probe [Fe(CN)6]−3/−4. As we mentioned above, the EIS method can sensitively monitor the changes of charge transfer resistance, Rct, due to the alterations at the sensor surface. The binding of negatively charged OTA to the aptamers resulted in increase of negative surface charge and in repealing of the redox probe from the surface. This causes increase in Rct value. At the same time other parameters of the circuit such are capacitance and Warburg impedance changed only slightly. The changes in Rct can serve as an analytical signal. This value sharply increased at relatively low OTA concentration range 0.1–3 nM with saturation at concentrations approx. 100 nM. This dependence had the shape of Langmuir isotherm and can be described by the equation analogical to that presented above (Eq. 10.1). The analysis of this dependence allowed to obtain KD = 8.3 ± 0.8 nM . This value is lower in comparison with that determined by fluorescence method for free aptamers in a volume  as well as that determined by acoustic method . But in EIS sensor the aptamers have been immobilized by different method, using chemisorption, which may affect the aptamers affinity properties. This biosensor exhibited comparable sensitivity with above-mentioned indirect assay (LOD 0.4 nM) and selectively detected OTA. The sensor was regenerable in 1 mM HCl and successfully validated in coffee and flour with recovery of 88% and 104%, respectively for spiked samples containing 10 nM OTA [81, 92]. Most recently the EIS biosensor based on DNA aptamer specific to OTA covalently immobilized onto mixed Langmuir–Blodgett monolayer composed of polyanilyne-stearic acid and deposited on ITO coated glass plates has been reported . This sensor revealed similar detection limit (0.24 nM), however the fabrication procedure has been more complicated in comparison with simple chemisorption used in our work. The sensor discriminated between OTA and aflatoxin, however it has not been validated in real food samples. In addition the Rct changes were of opposite direction, i.e. with increased OTA concentration the Rct value decreased at presence of redox probe. This is in contradiction with already published papers.
Thus, the aptasensors are of high perspective even for detection small molecules such are OTA. The sensitivity of detection in most cases is similar to those of antibodies. Substantial advantage of aptasensors is possibility of surface regeneration which allowing their multiple use. Moreover, recent papers on application SPR, thickness shear mode acoustic method and EIS are evidence of possibility of direct detection OTA without using OTA-protein conjugates or other labels.
The biosensor technology is certainly powerful tool for detection food mycotoxins such as ochratoxin A. The achievements reported in this review revealed that most of the approaches allowing detect OTA with high sensitivity, which is better than allowable contamination of food by this toxin. Rather perspective direction in biosensor development consisting in application of DNA aptamers. Using these novel biopolymers even most sensitive biosensor assay was demonstrated, allowing detection of OTA with LOD 0.25 pM. Direct detection methods such are acoustic and electrochemical impedance spectroscopy are rather challenging, due to fast response and high sensitivity which is substantial advantage over traditional methods such are HPLC or mass spectroscopy. We believe that further effort should result in appearance of low cost, portable and easy to use biosensor for detection OTA and other toxins applicable in food factories and agricultural farms.
The work was supported by the Slovak Research and Development Agency (contracts No. APVV-0410-10, LPP-0341-09).
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