Optical sensors for boron detection based on the use of 2,3,6,7,10,11-hexahydroxytr phenylene or its derivatives

Field of the inventiona

Field of the invention

The present invention relates generically to the use of optical sensors for the detection and quantification of boron in the forms of boric acid, tetrahydroxyborate, boronic acids and boronates. More specifically, it refers to the use of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) or its derivatives as optical sensors for the detection of boric acid, tetrahydroxyborate, boronic acids and boronates in aqueous media, from the variation of the intensity of fluorescence emission, with or without the use of an internal fluorescence standard, or from the variation in fluorescence lifetimes, or from the variation of the intensity of light absorption.

Background of the invention

Boron is a chemical element considered to be beneficial for human health and agriculture in trace quantities . [1, 2] However, in excess it becomes toxic both for human beings and crops . [1,2,3,4]

Boron containing compounds are used in several industrial applications, including the manufacture of soaps and detergents, glass and ceramics, insecticides, fertilizers, semiconductors and electronics industry, flame retardants, high-duress compounds, or in the pharmaceutical industry . [2, 3, 4] Therefore, a high boron content in water can result from residual waters discharges. [2, 3, 4] Boron can also exist naturally in high concentrations in water due to leaching from rocks and soils containing borates and borosilicates . [2, 3, 4] In drinkable water, boron is generally present in concentrations lower than 0.5 mg/L (0.5 ppm) . The World Health Organization recommends that the concentration of boron in water for human consumption should be lower than 2.4 mg/L (2.4 ppm). [3,4] The Portuguese and European Union legislation establish a maximum permissible value of 1.0 mg/L (1.0 ppm) of boron. [5,6] As far as crops are concerned, the tolerated boron amount depends on the species. Adverse effects have been observed in the most sensitive crops, such as lemon, even below 0.5 mg/L (0.5 ppm) of boron in irrigation water. [7]

The most sensitive standard methods for the determination of the boron content in water are inductively coupled plasma atomic emission spectroscopy (ICP-AES) , with a limit of detection between 6 to 10 (0.006 to 0.010 ppm) of boron, and inductively coupled plasma mass spectrometry (ICP-MS), with a detection limit of 0.15 pg/l (0.15 ppb) of boron. [3, 4, 8] However, these methods have several disadvantages. Their cost is high, they are not portable, and involve the use of complex equipment .

As an alternative, spectrophotometric, and more recently, spectrofluorimetric methods, have been described, using reagents that act as sensors. Both methods require low-cost and easy to operate equipment, and can be adapted for portable use. Although spectrofluorimetry has higher sensitivity than spectrophotometry, there are relatively few examples of spectrofluorimetric boron sensors . [9, 10]

An example of such a sensor is azomethine-H, which can be used both for spectrophotometric and spectrofluorimetric detection, with limits of detection of about 10 g/L (0.010 ppm), [4] being possibly the most common spectrophotometric method. [1, 4, 11, 12, 13] There are also other detection methods that use reagents with chemical structures similar to azomethine-H [14,15,16,17] or precursors of this reagent, salicylaldehyde and l-amino-8-naphthol-3, 6-disulfonate . [12, 18] Other spectrophotometry detection reagents are curcumin, [1, 8, 11, 19] carminic acid, [1, 8, 11] quinalizarin, [1, 8] 2, -dinitro-l, 8-naphthalenediol, [11] 2, 6-dihydroxybenzoic acid and malachite green, [11] 2 , 3-dihydroxynaphthalene and crystal violet, [11] mandelic acid and malachite green, [11] pyrocatechol derivatives and ethyl violet, [11] salicylate and ferroin, [11]

I, 1-diantrimide, [11] stilbene azo benzene sulfonate, [20] arsenazo and crystal violet. [8] For boron detection in the form of BF ₄ ^" it is possible to use methylene blue, [8,11] Nile blue A, [11] capri blue, [11] malachite green, [11] and chrompyrazole

II. [11]

Other reagents that can be used for spectrofluorimetric boron detection are alyzarin red S, [8,9] chromotropic acid, [8, 21, 22] 2, 3-dihydroxynaphthalene, [23, 24] and 1, 2-naphthoquinone-4- sulfonate . [25 ] Other fluorescent sensors for the detection of boronic acids with diethanolamine groups have been described, [26] but later contested. [27]

Other instrumental methods less widely used for boron detection include potentiometric methods, [8] flame atomic spectrometric methods (emission or absorption) , [8 ] high resolution mass spectrometry, [8] high performance liquid chromatography (HPLC) with ionic exchange columns, [8] neutron activation analysis (including neutron capture radiography and prompt-γ activation analysis) , [8 ] and dual-pulse laser-induced breakdown spectroscopy. [28]

2, 3, 6, 7 , 10, 11-hexahydroxytriphenylene (I), here proposed as a boron sensor, is a commercial compound which has been used as a synthetic precursor for discotic compounds used in liquid crystal mixtures, [29] and in the synthesis of bidimensional and tridimensional supramolecular structures and covalent organic frameworks. [30,31,32,33] In the present invention, the use of 2, 3, 6, 7 , 10, 11- hexahydroxytriphenylene (I) and its derivatives as sensors for boron, using spectrofluorimetric or spectrophotometric detection is proposed. The use of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) with spectrofluorimetric detection, in the experimental conditions described in the Detailed Description section, has limits of detection of about 0.01 ppm of boron in the case of boric acid or tetrahydroxyborate, of the same order of magnitude of the most sensitive spectrofluorimetric methods and ICP-AES. There is also the possibility of incorporating 2, 3, 6, 7, 10, 11- hexahydroxytriphenylene (I) and its derivatives in nanoparticles or polymers, providing an enhanced measuring procedure.

Summary of the invention

In accordance with what was previously exposed, the present invention relates to a new method for the detection of boron in the forms of boric acid and tetrahydroxyborate, as well as boronic acids and boronates, in aqueous media, using 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) or its derivatives.

2 , 3, 6, 7 , 10 , 11-hexahydroxytriphenylene (I) and its derivatives, onwards designated as "sensor", present properties which allow its use as an optical sensor of boron in the form of boric acid and tetrahydroxyborate in aqueous media, as well as boronic acids and boronates in aqueous media, from the variation of the intensity of fluorescence emission, with or without the use of an internal fluorescence standard, or from the variation in fluorescence lifetimes, or from the variation of the intensity of light absorption.

For the use of the sensor in the detection of boric acid and tetrahydroxyborate in aqueous media, the concentration range of these compounds should be between 0.01 and 1000 ppm of boron in the case of spectrofluorimetric detection, and between 5 and 1000 ppm of boron in the case of spectrophotometric detection, the pH should be between 7 and 10, and the working temperature should be kept constant between 5°C and 40°C throughout the calibration and sample measurements.

For the use of the sensor in the detection of boronic acids and boronates in aqueous media, the concentration range of these compounds should be between 0.006 and 1000 ppm of boron in the case of spectrofluorimetric detection and between 0.3 and 1000 ppm of boron in the case of spectrophotometry detection, the pH should be between 7 and 10, and the working temperature should be kept constant between 5°C and 40°C throughout the calibration and sample measurements.

The present invention is related to the use of 2, 3, 6, 7, 10, 11- hexahydroxytriphenylene (I) or its derivatives as optical sensors for the detection and quantification of boron in natural waters, waters for domestic, agricultural or industrial use, as well as domestic, agricultural or industrial effluents.

Other objectives and advantages of the present invention will become apparent from the following detailed description of the invention.

Detailed description of the invention

The present invention relates to a method for detection of boron in the forms of boric acid and tetrahydroxyborate, as well as boronic acids and boronates, in aqueous media, using 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I), illustrated in Figure 1, or its derivatives, illustrated in Figure 2, onwards designated as "sensor".

The sensor presents properties that allow its use as an optical sensor for the detection of boron in the forms of boric acid, tetrahydroxyborate, boronic acids and boronates in aqueous media, from the variation of the intensity of fluorescence emission, with or without the use of an internal fluorescence standard, or from the variation of fluorescence lifetimes, or from the variation of intensity of light absorption. Therefore, the sensor allows the detection by techniques of spectrofluorimetry, fluorescence lifetime measurement or ultraviolet-visible absorption spectrophotometry.

The sensor can coordinate with boric acid, tetrahydroxyborate, boronic acids and boronates, forming complexes that show fluorescence or ultraviolet-visible light absorption properties that are distinct in terms of intensity, spectra shape and/or fluorescence lifetime, of those of the free sensor.

The sensor can be used in the presence of a chelating agent in order to avoid the coordination of chemical species other than boric acid, tetrahydroxyborate, boronic acids and boronates, with the sensor, particularly to avoid the coordination of other metal ions.

The chelating agent should have null interaction with boric acid, tetrahydroxyborate, boronic acids and boronates, which means that the chelating agent, if used, should have a non- measurable interaction, or act without affecting the interaction of the sensor with boric acid, tetrahydroxyborate, boronic acids and boronates .

The chelating agent should not affect the sensor response, nor absorb light nor fluoresce in the range of wavelengths used to detect the sensor. The chelating agent should present transmittance higher than 50% in the range of wavelengths between 200 nm and 390 nm, and fluorescence intensity lower than 5% of the fluorescence intensity of the sensor in the absence of boron, in a range of wavelengths where there might be overlap of the emission wavelengths of both. The chelating agent must be soluble in water, or immobilized on particles or polymers stable in aqueous dispersion.

An example of a chelating agent with these characteristics is ethylenediaminetetraacetic acid (EDTA) .

The use of the sensor for detection of boric acid and tetrahydroxyborate in aqueous media can be made in a concentration range between 0.01 and 1000 ppm of boron in the case of spectrofluorimetric detection, and between 5 and 1000 ppm of boron in the case of spectrophotometry detection, when the sensor is present in a concentration of 1 μΜ, at pH 9.1 and temperature of 23°C. The pH should be between 7 and 10, and the temperature kept constant between 5°C and 40°C. A chelating agent can be used to limit the interference by other metal ions present in solution.

The use of the sensor for detection of boronic acids and boronates in aqueous media can be made in a concentration range between 0.006 and 1000 ppm of boron in the case of spectrofluorimetric detection, and between 0.3 and 1000 ppm of boron in the case of spectrophotometric detection, when the sensor is present in a concentration of 1 μΜ, at pH 9.1 and 23°C. The pH should be between 7 and 10, and the temperature kept constant between 5°C and 40°C. A chelating agent can be used to limit the interferences by other metal ions present in solution.

Boronic acids and boronates that can be detected by the sensor include phenylboronic acid, other polyaromatic boronic acids, aliphatic boronic acids with different chain length, and their derivatives functionalized on the aromatic ring or in the aliphatic chain with groups of carboxylic acid, hydroxyl, halogen, thiol, carbonyl, alkyl, aryl, sulfonic, primary, secondary or tertiary amines, nitro, ether, ester, as well as their respective conjugated bases.

The derivatives of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene that can be used as optical sensors include the compounds substituted on the aromatic ring or on the hydroxyl groups through a spacer with one or more carbon atoms, by terminal groups of amine, alkoxysilane, acrylate, methacrylate, vinyl, styrene, epoxy, thiol, azide, succinimide, isocyanate, thiocyanate, alkyne, hydroxyl, carbonyl, ester, ether or carboxylate, which possess at least two vicinal hydroxyl groups, as illustrated on Figure 2.

The sensor and its derivatives are soluble in water; or in alternative might be immobilized in polymers that are stable in water, more preferably polymers soluble in water; or might be immobilized in polymeric nanoparticles, metallic nanoparticles or inorganic nanoparticles, of dimensions comprised between 3 to 1000 nm yielding supported sensors that are stable in aqueous dispersion .

The sensor presents properties as a sensor of boron in the forms of boric acid, tetrahydroxyborate, boronic acids or boronates in aqueous solution, from the measurement of the variation of fluorescence intensity, with or without internal calibration, or the measurement of the variation of fluorescence lifetimes. When the sensor is interrogated by measuring the variation of fluorescence intensity, there is the possibility of using a second fluorescent compound as internal fluorescence standard.

The internal fluorescence standard, if used, should not interact with boric acid, tetrahydroxyborate, boronic acids or boronates, which means that the internal fluorescence standard, if used, should have a non-measurable interaction with boric acid, tetrahydroxyborate, boronic acids or boronates, or act without affecting the interaction of the sensor with boric acid, tetrahydroxyborate, boronic acids and boronates.

The internal fluorescence standard can be interrogated using excitation wavelengths between 200 nm and 390 nm, and should emit fluorescence preferably at wavelengths where there is no overlap with the fluorescence of the sensor, or where the wavelength overlap with the sensor in the absence of boron is below 5%, with greater preference above 600 nm. The fluorescence internal standard should be soluble in water, or immobilized in particles or polymers that are stable in aqueous dispersion, should have a high fluorescence quantum yield, preferably above 0.1, as well as a high molar extinction coefficient, preferably higher than 1000 M ^-1 cm ^_1 .

An example of a compound with these characteristics and that can be used as an internal fluorescence standard is sulforhodamine 101.

In the case of an internal fluorescence standard being used, the internal calibration should be made by comparison of the fluorescence intensities of the internal fluorescent standard with the fluorescence intensity of the sensor or its derivatives. The excitation wavelength used to interrogate the sensor can vary between 200 nm and 390 nm. The internal fluorescence standard, if used, can be excited in the same wavelength range. The wavelength where the fluorescence emission of the free sensor or its complexes with boric acid or tetrahydroxyborate is observed can vary between 350 nm and 600 nm, more preferably in the region between 360 nm and 430 nm. The wavelength where emission of the sensor complexes with boronic acids or boronates can also vary between 350 nm and 600 nm, with greater preference in the region between 360 nm and 430 nm. The radiation source appropriate to the excitation of the sensor and the internal fluorescence standard, when used, for measurement of the variation of fluorescence intensity of the sensor, must preferably allow excitation in wavelengths between 200 nm and 390 nm. The excitation and emission wavelengths can be selected with a prism, diffraction grating, filter or another device with a similar function. The angle between the excitation beam and the detection of emission can vary between 0° and 180°.

When the sensor is interrogated by measurement of its fluorescence lifetime, the excitation wavelength can vary between 200 nm and 390 nm. The wavelength where the emission of the free sensor or its complexes with boric acid or tetrahydroxyborate is observed can vary between 350 nm and 600 nm, preferably in the region between 360 nm and 430 nm. The emission wavelength of the sensor complexes with boronic acids or boronates can also vary between 350 nm and 600 nm, preferably in the region between 360 nm and 430 nm.

The pulsed or modulated frequency radiation source to the excitation of the sensor for fluorescence lifetime measurements, should preferably allow excitation in wavelengths between 200 nm and 390 nm. The excitation and fluorescence emission wavelengths can be selected with a prism, diffraction grating, filter or another device with a similar function. The angle between the excitation beam and the detection of emission can vary between 0° and 180°.

When the sensor is interrogated by measurement of the variation of the amount of absorbed or transmitted light, the wavelength can vary between 250 nm and 600 nm, preferably in the region between 270 nm e 290 nm in order to detect the complexes of the sensor with boric acid or tetrahydroxyborate, and between 290 nm e 350 nm to detect the complexes of the sensor with boronic acids or boronates. The radiation source appropriate to the measurement of the amount of absorbed or transmitted light by the sensor should preferably allow the illumination with wavelengths between 250 nm and 600 nm, preferably in the region between 270 nm and > 290 nm to detect the complexes of the sensor with boric acid or tetrahydroxyborate, and between 290 nm e 350 nm to detect the complexes of the sensor with boronic acids or boronates.

The present invention is related to the use of 2,3,6,7,10,11- ) hexahydroxytriphenylene (I) or its derivatives as optical sensors for the detection and quantification of boron in natural waters, waters for domestic, agricultural or industrial use, as well as domestic, agricultural or industrial effluents. i Description of the figures

Figure 1 presents the structure of 2,3,6,7,10,11- hexahydroxytriphenylene (I) .

I Figure 2 presents the structure of the 2,3,6,7,10,11- hexahydroxytriphenylene (I) derivatives that can be used as boron sensors. Ri, -R ₂ , -R3 and -R represent groups -O-R or - (CH ₂ ) _n -R, with n>l. -R5, -R6, -R7, -Rs/ -R9 and -Rio represent -H or -(CH ₂ )n- _/ ' with n>l . R represents a hydrogen atom or one of the following groups: amine, alkoxysilane, acrylate, methacrylate, vinyl, styrene, epoxy, thiol, azide, succinimide, isocyanate, thiocyanate, alkyne, hydroxyl, carbonyl, ester, ether or carboxylate .

Figure 3 presents the fluorescence emission titration curves, as a function of pH, of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) with the recommended analysis interval marked as a gray rectangle. The y-coordinate refers to the fluorescence intensity at a wavelength of 388 nm, expressed in arbitrary units, and the x-coordinate refers to pH values. The spectra were measured at an excitation wavelength interval between 307 and 313 nm, at 23°C, in the presence of EDTA (10 itiM) . A: Titration in the absence of boron; B: Titration in the presence of boric acid tetrahydroxiborate (1 mM) ; C: Titration in the presence of phenylboronic acid / phenylboronate (1 mM) .

Figure 4 presents the emission spectra of 2,3,6,7,10,11- hexahydroxytriphenylene (I) (1 μΜ) in presence of a mixture of boric acid / tetrahydroxyborate at different concentrations. The y-coordinate refers to the intensity of fluorescence, expressed in arbitrary units, and the x-coordinate refers to wavelength values, expressed in nanometers. The spectra were measured at an excitation wavelength between 307 nm and 313 nm, at 23 °C and pH 9.1, in the presence of EDTA (10 mM) . An increase in the emission intensity of the sensor can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with boric acid / tetrahydroxyborate. A: 54 ppm of boron; B: 11 ppm of boron; C: 1.1 ppm of boron; D: absence of boron.

Figure 5 presents the emission spectra of 2,3,6,7,10,11- hexahydroxytriphenylene (I) (1 μΜ) in presence of a mixture of phenylboronic acid / phenylboronate at different concentrations. The y-coordinate refers to the intensity of fluorescence, expressed in arbitrary units, and the x-coordinate refers to wavelength values, expressed in nanometers. The spectra were measured at an excitation wavelength between 307 nm and 313 nm, at 23°C and pH 9.1, in the presence of EDTA (10 mM) . An increase in the emission intensity of the sensor can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with phenylboronic acid phenylboronate. A: 54 ppm of boron; B: 11 ppm of boron; C: 1.1 ppm of boron; D: 0.11 ppm of boron; E: absence of boron. Figure 6 presents the spectrofluorimetric titration curves of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) as a function of the boro concentration in a mixture Of boric acid / tetrahydroxyborate . The figure has an expansion of the spectrofluorimetric titration curve in the lower concentrations region. Both in the main figure and in the expansion figure, the y-coordinate refers to the fluorescence intensity, expressed in arbitrary units, and the x-coordinate refers to boron concentration values, expressed in ppm of boron. The spectra were measured at excitation wavelength between 307 and 313 ran, at 23°C and pH 9.1, in the presence de EDTA (10 mM) . An increase in the emission intensity of the sensor can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with boric acid / tetrahydroxyborate.

Figure 7 presents the spectrofluorimetric titration curves of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) as a function of the global concentration of mixture of phenylboronic acid / phenylboronate . The figure has an expansion of the spectrofluorimetric titration curve in the lower concentrations region. Both in the main figure and in the inserted figure, the y-coordinate refers to the fluorescence intensity, expressed in arbitrary units, and the x-coordinate refers to boron concentration values, expressed in ppm of boron. The spectra were measured at excitation wavelength between 307 and 313 ran, at 23°C and pH 9.1, in the presence de EDTA (10 mM) . An increase in the emission intensity of the sensor can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with phenylboronic acid / phenylboronate .

Figure 8 presents the ultraviolet-visible light absorption spectra of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) in the presence of mixture of boric acid / tetrahydroxyborate at W

14 different concentrations. The y-coordinate refers to the absorbance and the x-coordinate refers to wavelength values, expressed in nanometers. The spectra were measured at 23 °C and pH 9.1, in the presence of EDTA (10 mM) . An increase in light absorption, especially visible at 270-290 nm, can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with boric acid / tetrahydroxyborate . A: 54 ppm of boron; B: 11 ppm of boron; C: 1.1 ppm of boron; D: 0.11 ppm of boron; E: absence of boron.

Figure 9 presents the ultraviolet-visible light absorption spectra of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) in the presence of mixture of phenylboronic acid / phenylboronate at different concentrations. The y-coordinate refers to the absorbance and the x-coordinate refers to wavelength values, expressed in nanometers. The spectra were measured at 23 °C and pH 9.1, in the presence of EDTA (10 mM) . An increase in light absorption, especially visible at 290-350 nm, can be observed upon increasing of boron concentration. This is due to the coordination of the sensor with phenylboronic acid / phenylboronate. Below 290 nm, the absorption of phenylboronic acid or phenylboronate is observed. A: 54 ppm of boron; B: 11 ppm of boron; C: 1.1 ppm of boron; D: 0.11 ppm of boron; E: absence of boron; F: mixture of phenylboronic acid / phenylboronate at a total boron concentration of 54 ppm, in the absence of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I).

Figure 10 presents the spectrophotometric titration curve of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) as a function of the global concentration of mixture of boric acid / tetrahydroxyborate. The y-coordinate refers to the absorbance at the wavelength of 285 nm and the x-coordinate refers to boron concentration values, expressed in ppm of boron. The spectra were measured at 23 °C and pH 9.1, in the presence de EDTA (10 mM) . An increase in light absorption can be observed upon increasing of boron concentration. This is due to the coordination with boric acid / tetrahydroxyborate .

Figure 11 presents the spectrophotometry titration curve of 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) (1 μΜ) as a function of the global concentration of mixture of phenylboronic acid / phenylboronate . The y-coordinate refers to the absorbance at 300 nm and the x-coordinate refers to boron concentration values, expressed in ppm of boron. The spectra were measured at 23°C and pH 9.1, in the presence de EDTA (10 mM) . An increase in light absorption can be observed upon increasing the boron concentration, due to the coordination with phenylboronic acid / phenylboronate .

Examples

The fluorescence emission titration curves as a function of pH for 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (I) in the absence of boron and in the presence of boric acid / tetrahydroxyborate (1 mM) or in the presence of phenylboronic acid / phenylboronate (1 mM) are represented in Figure 3. Differences between pH 7 and 10 may be observed.

The recommended pH interval for boron detection, where the emission of the non-coordinated sensor is residual and the emission of the boron sensor complexes is significant, is between pH 8.5 and 9.5. This interval has been marked in Figure 3 with a grey rectangle. The spectra have been measured at excitation wavelengths between 307 nm and 313 nm, 23 °C, and in the presence of EDTA (10 mM) .

The experiments of boron detection in aqueous solution, both for boric acid or tetrahydroxyborate, and for phenylboronic acid or phenylboronate, were performed at room temperature, 23°C. The sensor was used at a concentration of 1 μΜ. The pH was kept constant at 9.1 using ammonia buffer (20 mM) . The measurements were performed in the presence of EDTA (10 mM) which acted as a chelating agent.

Spectre-fluorimetric detection from the variation of the fluorescence intensity

The boron spectrofluorimetric detection experiments were performed using a 450 xenon lamp as excitation source. The excitation wavelength interval was from 307 nm to 313 nm, selected with a monochromator . The angle between the incident light beam and the collected emission was of approximately 90°, with the emitted light also being selected with a monochromator. The fluorescence intensity was measured between 330 nm and 600 nm.

The emission spectra of the sensor at different concentrations of the mixtures boric acid / tetrahydroxyborate and phenylboronic acid / phenylboronate are presented in Figures 4 and 5, respectively. In both cases the emission intensity increases with the increase in boron concentration, due to the coordination of the sensor with boron. The spectrofluorimetric sensor titration curves as a function of the concentration of the mixtures boric acid / tetrahydroxyborate and phenylboronic acid / phenylboronate are presented in Figures 6 and 7, respectively. The limit of detection was determined in the aforementioned experimental conditions as 0.01 ppm of boron in the case of boric acid / tetrahydroxyborate, and 0.006 ppm of boron in the case of phenylboronic acid or phenylboronate.

Spectrophotometric detection from the variation of the ultraviolet-visible absorbance The boron detection experiments through spectrophotometry at ultraviolet-visible wavelengths were performed using deuterium and tungsten lamps as light sources. The absorbance was detected between 250 nm and 500 nm.

The absorption spectra of the sensor at different concentrations of the mixtures boric acid / tetrahydroxyborate and phenylboronic acid / phenylboronate are presented in Figures 8 and 9, respectively. In both cases, the absorbance increases with the increase in boron concentration, due to the coordination of the sensor with boron. The spectrophotometric sensor titration curves as a function of the concentration of the mixtures boric acid / tetrahydroxyborate and phenylboronic acid / phenylboronate are presented in Figures 10 and 11, respectively. The limit of detection was determined in the aforementioned experimental conditions, as 5 ppm of boron in the case of boric acid / tetrahydroxyborate, and 0.3 ppm of boron in the case of phenylboronic acid / phenylboronate.

References

[1] ZAIJUN, Li, ZHENGWEI, Cui, JIAN, Tang. The determination of boron in food and seed by spectrophotometry using a new reagent 3, -dihydroxyazomethine-H. Food Chemistry 94: 310-314, 2006.

[2] BATAYNEH, A. T. Toxic (aluminum, beryllium, boron, chromium and zinc) in groundwater- health risk assessment. Int. J. Environ. Sci. Technol. 9: 153-162, 2012.

[3] WORLD HEALTH ORGANIZATION. Guidelines for drinking-water quality. Fourth edition. World Health Organization, 2011.

[4] WORLD HEALTH ORGANIZATION. Boron in drinking-water.

Background document for development of WHO Guidelines for

Drinking-water Quality. World Health Organization, 2009.

[5] Decreto-Lei n.° 306/2007 de 27 de Agosto. Estabelece o regime da qualidade da agua destinada ao consumo humano. Ministerio do Ambiente, do Ordenamento do Territorio e do Desenvolvimento Regional. Diario da Repiiblica, l. ^a serie, N.° 164, p. 5747. 27 de Agosto de 2007.

[6] Council Directive 98/83/EC of 3 November 1998. On the quality of water intended for human consumption. Official Journal of the European Communities, p. 330/32, 5/12/98

[7] AYERS, R. S., WESTCOT, D. W. Water quality for agriculture. FAO Irrigation and Drainage Paper 29 Rev. 1. Rome: Food and Agriculture Organization of the United Nations, 1985.

[8] SAH, Ram . , BROWN, Patrick H. Techniques for boron determination and their application to the analysis of plant and soil samples. Plant and Soil 193 (1-2): 15-33, 1997.

[9] CAMPANA, Ana M. Garcia, BARRERO, Fermin Ales, CEBA, Manuel Roman. Spectrofluorimetric Determination of Boron in Soils, Plants and Natural Waters With Alizarin Red S. Analyst 117: 1189-1198, 1992.

[10] WHITE, Charles E., WEISSLER, Alfred, BUSKER, David. Fluorometric Determination of Microgram Quantities of Boron. Anal. Chem. 19: 802, 1947.

[11] MARCZENKO, Zygmunt, BALCERZAK, Maria. Separation, preconcentration, and spectrophotometry in inorganic analysis. Amsterdam: Elsevier Science Ltd, 2000.

[12] HARP, D. L. Modifications to the azomethine-H method for determining boron in water. Analytica Chimica Acta 346 (3) : 373- 379, 1997.

[13] MERDIVAN, Erinc, BENIBOL, Yalim, SEYHAN, Serap. Fluorimetric detection of boron by azomethine-H in micellar solution and sol-gel. Spectrochimica Acta Part A 71 (5): 2045- 2049, 2009.

[14] ZHIRKOV, A. A., RAZVAZHNAYA, 0. V., KAZAKOVA, T. A., PETRENKO, D. B., PROSKURNIN, M. A., DEDKOV, Yu. M. , ZUEV, B. K. Quantification of boron in water by of spectrophotometry and thermal lens spectrometry using reaction with beryllon III. Journal of Analytical Chemistry, 66 (12): 1180-1185, 2011. [15] CHINA Inst. Atomic Energy. Spectrophotometry method for measuring boron content in BNCT medicament in biological samples. DENG, Xinrong; FAN, Caiyun; LI, Fenglin; LIU, Zihua; LUO, Zhifu. Int. CI. G01N21/31; G01N21/33; G01N21/78, CN101713740 . 2010-05-26.

[16] CHUGOKU Electric Power Co Inc., CHUDEN Kankyo Technos Co Ltd. Quantitative Analyzing Method of Boron. HIROSHI, Kubota, SHINJI, Fukunaga, EMIKO, Nishiyama. Int. CI. GOlNl/10; G01N1/22; G01N21/59, JP2005291971. 2005-10-20.

[17] TAKAOKA Kasei Kogyo KK, KYUSHU Univ. Boron Analyzing Method. KAZUHISA, Yoshimura, KO, Takehara, KEI, Inagi, TAKASHI, Higashihara. Int. CI. G01N27/416; G01N27/48, JP2011163888. 2011- 08-25.

[18] TAKAHASHI, T, YAWATA, S, HOSHINO, H. Determination of boron in water samples at nanograms per cubic decimeter levels by reversed-phase partition high-performance liquid chromatography with precolumn complexation reaction using salicylaldehyde and l-amino-8-naphthol-3 , 6-disulfonate . Analytical and

Bioanalytical Chemistry, 391 (3): 1101-1106, 2008.

[19] HANGAVEL, S., DHAVILE, S. M., DASH, K. , CHAURASIA, S. C. Spectrophotometric determination of boron in complex matrices by isothermal distillation of borate ester into curcumin, Analytica Chimica Acta, 502: 265-270, 2004.

[20] BAI Li. Boron determination solution and colorimetric determination tube therefor. SHUHUA, Guo Li, ZHENGRANG, Guo, YUN, Chen, JING, Zhou, HAIMIN, Wang, WUYI, Liu. Int. CI. G01N21/03; G01N21/27; G01N21/31; G01N21/78, CN1766561, CN100487426

[21] LAPID, J., FARHI, S., KORESH, Y. Spectrofluorometric Determination Of Boron With Chromotropic-Acid. Analytical Letters, 9 (4): 355-360, 1976.

[22] KUEMMEL, D. F. , MELLON, M. G. Ultraviolet Absorptiometry Determination of Boron in Aqueous Medium Using Chromotropic Acid. Anal. Chem. , 29 (3): 378-382, 1957. [23] IWATA, Junichi, WATANABE, Kunihiro, ITAGAKI, Masayuki. Flow injection fluorometric determination of boron in steels with 2, 3-dihydroxynaphthalene. Bunseki Kagaku 55 (7): 473-480, 2006.

[24] SAKATA, Shigeya, IWATA, Junichi, SHITANDA, Isao, ITAGAKI, Masayuki, WATANABE, Kunihiro. Flow Injection Fluorometric Determination of Boron in Steel with 2 , 3-Dihydroxynaphthalene Using Methanol as a Carrier Liquid. Tetsu To Hagane-Journal Of The Iron And Steel Institute Of Japan, 97 (2): 65-69, 2011.

[25] MA, Linjin, ZHANG, Zhenxuan, LI, Quanmin. Spectrophotometric determination of boron based on charge transfer reaction. Spectrochimica Acta Part A, 79: 599-602, 2011.

[26] SPRINGSTEEN, Greg, BALLARD, C. Eric, GAO, Shouhai, WANG, Wei, WANG, Binghe. The Development of Photometric Sensors for Boronic Acids. Bioorganic Chemistry 29: 259-270, 2001

[27] ARIMORI, Susumu, WARD, Christopher J., JAMES, Tony D. The first fluorescent sensor for boronic and boric acids with sensitivity at sub-micromolar concentrations — a cautionary tale. Chem. Commun. , 2001: 2018-2019, 2001.

[28] LEE, Dong-Hyoung, HAN, Sol-Chan, KIM, Tae-Hyeong, YUN, Jong-Il. Highly Sensitive Analysis of Boron and Lithium in Aqueous Solution Using Dual-Pulse Laser-Induced Breakdown Spectroscopy. Anal. Chem., 83 (24): 9456-9461, 2011.

[29] SECRETARY of State for Defence Brit., DEFENCE Research Agency. Discotic compounds for use in liquid crystal mixtures. GOODBY John William, HIRD, Michael, BEATTIE, David Richard, HINDMARSH, Paul, GRAY, George William, MCDONNELL, Damien Gerard, JONES, John Clifford, PHILLIPS, Timothy Jonathan. United Kingdom, Int. CI. C07C255/54, C07C255/55, C07C255/57, C07C43/20, C07C43/205, C07C43/215, C07C43/225, C07C69/00, C07C69/76, C07C69/773, C07C69/92, C09K19/32, G02F1/13, (IPCl-7): C07C43/20, C07C69/76, C09K19/32, G02F1/13, Eur. CI. C07C43/20; C07C69/92; C09K19/32, EP0702668, W09429263, US5750050, JPH08511512, GB2294046, DE69320655. 1996-03-27. [30] DOGRU, Mirjam; BEIN, Thomas. Covalent Organic Frameworks: Growing honeycombs on grapheme. Nature nanotechnology 6 (6):333- 335, 2011.

[31] EL-KADERI, Hani M. , HUNT , Joseph R. , MENDOZA-CORTES, Jose L., COTE, Adrien P., TAYLOR, Robert E., O'KEEFFE, Michael, YAGHI, Omar M. Designed Synthesis of 3D Covalent Organic Frameworks. Science 316: 268-272, 2007.

[32] THEBAULT, Frederic, OHRSTROM, Lars, HAUKKA, Matti. 2, 3, 6, 7, 10, 11-Hexahydroxytriphenylene tetrahydrate : a new form of an important starting material for supramolecular chemistry and covalent organic frameworks. Acta Crystallographica Section C, 67 (4): ol43-ol45, 2011.

[33] FYFE, M. C. T., LOWE, J. . , STODDART, J. F. , WILLIAMS, D. J. An Extremely Stable Interwoven Supramolecular Bundle. Org. Lett. 2: 1221-1224, 2000.