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United States Patent 3,904,373
Harper September 9, 1975

Indicators covalently bound to insoluble carriers


Indicators insolubilized by covalent bonding to inorganic carriers for indicating the hydrogen ion concentration, oxidation-reduction state or specific ion concentration in liquid media. Since the indicators are insoluble they do not contaminate the solution tested, and they may be used repeatedly in different media. They replace the well known indicator test papers which consists of a substrate dyed with an indicator. Methods of making the indicators are also provided.

Inventors: Harper; Gerald Bruce (Toronto, CA)
Appl. No.: 05/409,876
Filed: October 26, 1973

Current U.S. Class: 422/425 ; 436/166; 436/169
Current International Class: G01N 31/22 (20060101); G01n 029/02 (); G01n 031/00 (); G01n 033/00 ()
Field of Search: 252/408 23/253TP 424/7

References Cited

U.S. Patent Documents
2626855 January 1953 Hand
2929829 March 1960 Morehouse
3350175 October 1967 McConnaughey et al.

Other References

"New Method Makes Possible Nonbleeding Indicator Paper", Chem. and Engin. News, Vol. 48, No. 10, p. 38 (Mar. 9, 1970). .
"Surface-Produced Alignment of Liquid Crystals", Kahn, F. J., et al., Proc. of The IEEE, Vol. 61, No. 7, pp. 823-828 (July 1973)..

Primary Examiner: Padgett; Benjamin R.
Assistant Examiner: Gron; T. S.
Attorney, Agent or Firm: Rutherford; F. Campbell


What is claimed is:

1. An insolubilized bound indicator useful in determining parameters such as hydrogen ion concentration (p.sup.H), oxidation-reduction potential or specific ion concentrations in solutions consisting of an organic indicator covalently coupled by means of an organo-functional silane coupling agent to an inorganic carrier having available hydroxyl or oxide groups.

2. A bound indicator as claimed in claim 1 wherein said silane coupling agent is combined with said indicator by means of an alkyl linkage.

3. A bound indicator as claimed in claim 1 wherein said silane coupling agent is combined with said indicator by means of an azo linkage.

4. A bound indicator as claimed in claim 1 wherein said silane coupling agent is combined with said indicator by means of a sulfonamide linkage.

5. A bound indicator as claimed in claim 1 wherein said silane coupling agent is represented by the general formula

X.sub.n SiR.sub.(4.sub.-n)

in which X represents a substituted or unsubstituted aryl, alkyl or alkyl-aryl group, the substituent(s) being selected from groups which include hydroxy, lower alkoxy, amino, lower alkylamino, lower dialkylamino, alkyl, nitro, nitroso, diazo, cyano, isocyano, isothiocyano, carboxy, carbonyl, keto, halocarbonyl, sulfoxy and halosulfonyl; R represents a group which may be lower alkoxy, aryloxy or halogen; and n is one of the integers, 1, 2 and 3.

6. A bound indicator as claimed in claim 1 wherein said carrier is a glass.

7. A bound indicator as claimed in claim 1 wherein said carrier is zirconia coated glass.

8. A bound indicator as claimed in claim 1 wherein said carrier is a metal oxide.

9. A bound indicator as claimed in claim 1 wherein said carrier is nickel oxide.

10. A bound indicator as claimed in claim 1 wherein said organic indicator is an N,N-dialkylaniline.

11. A bound indicator as claimed in claim 1 wherein said organic indicator is selected from the group consisting of diazo, amino, carboxy and alkylsilane derivatives of N,N-dimethylaniline.

12. A bound indicator as claimed in claim 1 wherein said organic indicator is selected from the group consisting of B-naphthol and derivatives thereof.

13. A bound indicator as claimed in claim 1 wherein said organic indicator is selected from the group consisting of triarylmethyl compounds.

14. A bound indicator as claimed in claim 1 wherein said organic indicator is phenolphthalein.

15. A bound indicator as claimed in claim 1 wherein said indicator is methylene blue.

16. A bound indicator as claimed in claim 1 wherein said organic indicator is Eriochrome Black T.

17. A process for the preparation of the bound indicator of claim 1 which includes the steps of providing a silane which has an organofunctional group, causing the silane to react with a carrier which has available hydroxyl or oxide groups, and causing the organofunctional group of the silane to react with an indicator.

This invention consists of a new type of indicators, which are hereinafter referred to as bound indicators.

Indicators are organic compounds which absorb visible light, which absorption(s) change in wave length and/or intensity as the composition of a solution to which the indicator may be exposed changes. Indicators are widely used to determine such parameters in solutions as hydrogen ion concentration (pH), oxidation-reduction potential or specific ion concentrations.

The term "indicator", as used herein, refers to organic molecules or ions which absorb visible light and whose absorption(s) change in wave length and/or intensity as solution conditions are varied, and includes "precursors", organic species which are not indicators as free species, but which when bound to inorganic carriers via silane coupling agents give a bound indicator. The term "bound indicator", as used herein, refers to any complex comprising an organic species covalently coupled via a silane coupling agent to a carrier having available hydroxyl or oxide groups, which absorb visible light and which absorption(s) change in wave length and/or intensity as solution conditions are varied.

Conventionally, indicators have been used by dissolving the indicator chemical entity in the liquid to be tested, or by coating a carrier, such as paper, with the chemical entity and then contacting the coated carrier with the liquid to be tested.

In whatever form indicators were previously used, a quantity of the indicator was required for each test. If a liquid was being monitored for changes in pH, ion concentration or oxidation-reduction potential with an indicator, samples of the liquid to be tested had to be contacted with the appropriate indicator and the sample then had to be discarded because it has been contaminated by the indicator or, in the case of a coated paper indicator, because the paper had absorbed a component of the test liquid. Thus the amount of previously known indicators consumed in an active chemical laboratory, or for control purposes in a plant, could be substantial. Such use could also be costly since a typical indicator species is expensive to prepare.

My invention consists of insoluble bound indicators which are able to fulfill the various functions of soluble indicators. Bound indicators have several advantages over soluble indicators:

1. they can, in most cases, be used repeatedly

2. they are largely unsusceptible to microbial attack

3. they are insoluble and hence do not contaminate systems

4. they may be made in a form which is especially convenient for laboratory operations, e.g. glass stirring rods

Bound indicators are very useful in analytical procedures in laboratories and industry and may also be used in the preparation of many foodstuffs, chemicals and pharmaceuticals. I have observed continued and apparently constant activity, as indicated by the intensity of colour changes, over a period of months, upon exposure to various organic and aqueous assay conditions. Because of their advantages, bound indicator sintered glass rods and the like are preferable to pH papers and soluble indicators for many uses. While bound indicators may be made with either inorganic or organic carriers, the former are normally preferable for use because they are more rigid and insoluble and more resistant to microbial attack.

Bound indicators must be substantially insoluble in a solution to be useful in it. Most indicators in widespread use change colour with pH and are, in fact, used to measure the pH of solutions. Other indicators have different functions, e.g. the measurement of the oxidation-reduction potential of a system, and the detection and measurement of various ions in solution.

The silane coupling agents are molecules which are characterized by two different kinds of reactivity. These are organofunctional, and silicon-functional, so characterized that the silicon portion of the molecule has an affinity for inorganic materials, such as glass and aluminum silicate, while the organic portion of the molecule is an indicator or precursor or is tailored to combine with indicators or precursors. One function of the coupling agent then, is to provide a bond between the indicator and the carrier. The variety of possible organofunctional silanes useful in this invention is limited only by the number of organofunctional groups which bind to silicon to give a stable coupling agent, by the stability of the bonds to the carrier and to the indicator and by the available sites in the organic species which yield an active bound indicator.

Many different silane coupling agents of the general formula X.sub.n SiR.sub.(4.sub.-n) can be used, wherein X is a substituent, which may be a substituted (or unsubstituted) aryl, alkyl or lower alkyl-aryl, nitro, nitroso, diazo, cyano, isocyano, isothiocyano, carboxy, carbonyl, keto, halocarbonyl, sulfoxy, sulfonyl halide, or more complex derivatives of any of these; R is a member selected from a group comprising lower alkoxy, phenoxy and halo; and n is an integer which is 1, 2 or 3, usually 1. The silane coupling agent may or may not itself be an indicator. This definition includes simple silane coupling agents wherein X is simply amino, carboxyl, carbonyl, sulfhydryl or halocarbonyl.

Coupling agents include gamma-aminopropyltriethoxysilane, 2,4,6-trimethoxybenzyltriethoxysilane, N-beta-aminoethyl-gamma-aminopropyltrimethoxysilane, and N-beta-aminoethyl-(alpha-methyl-gamma-aminopropyl)-dimethoxymethylsilane. While some simple silane coupling agents are commercially available, many others, including more complex ones, may be made by known chemical methods. For example, I have added 2,4,6-trimethoxybenzoic acid to trichlorosilane in acetonitrile, then added tri-n-propylamine to form 2,4,6-trimethoxybenzyltriethoxysilane. Ethanolysis yielded the useful coupling agent 2,4,6-trimethoxybenzyltriethoxysilane. As another example, gamma-aminopropyltriethoxysilane couples to inorganic carriers giving the aminoalkylsilane derivative, which can be reacted with alkoxybenzoyl chlorides to form another complex which binds diazotized indicators or precursors. Another reaction sequence involves reacting the aminoalkylsilane derivative with p-nitrobenzoyl chloride, reducing the nitro group with sodium dithionite and diazotizing with sodium nitrite: this diazonium salt attacks activated aryl rings of indicators or indicator precursors. Alternatively, an aminoalkylsilane derivative may be reacted with thiophosgene to give an isothiocyanoalkyl derivative which binds amino groups. Where the indicator or precursor or derivative contains a suitable aromatic ketone, aldehyde, acyl chloride or carboxy group it is possible to prepare silane coupling agents which are also indicators or precursors.

The carriers used can be organic, but generally, inorganic materials with available hydroxyl or oxide groups are preferred. The quantity of indicator or precursor which can be bound, and hence the colour intensity of the bound indicator, increases with increasing surface area of the carrier. Hence, a carrier such as smooth, unetched glass is an unsatisfactory carrier, as it yields a bound indicator of weak colour. The carriers must have little or no solubility in various solutions and are either weak acids or weak bases. They may also be classified in terms of chemical composition as siliceous materials, as non-siliceous metal oxides, or as mixtures of the two, such as zirconia-clad glass. Of the siliceous materials, the preferred carriers are sintered, etched or porous glass. These may be used in such forms as rods or discs, or as fragments. Glass has the advantages of being dimensionally stable, of being transparent or white in colour thus allowing colour changes to be easily judged, and it can be thoroughly cleaned to remove contaminants as, for example, by sterilization. The corrosion rate of glass varies with glass composition and solution conditions, but corrosion has remained undetectable throughout this work. Other useful siliceous inorganic carriers are silica gel, coloidal silica (commercially available under the trade mark Cab-O-Sil), wollastonite (a naturally-occuring calcium silicate) and bentonite. Representative non-siliceous metal oxides include alumina, hydroxy apatite and nickel oxide. These inorganic carriers may be classified as in Table I.

TABLE I ______________________________________ Inorganic Carriers Transition Metal Non-siliceous Metal Siliceous Oxides Oxides ______________________________________ Amorphous Crystalline Acid MeO Base MeO Glasses Silica Bentonite NiO Al.sub.2 O.sub.3 Hydroxy Gel apatite Coloidal Wollastonite Silica ______________________________________

Bound indicators may be classified under three general headings:

1. pH indicators

2. redox indicators (i.e. oxidation-reduction indicators)

3. adsorption indicators (i.e. ion detectors) of which examples of each class are listed below.

Bound pH indicators can be produced using many pH indicators or functionalized derivatives of those indicators. Suitable organic species include: phenolphthalein, fluorescein, phenol red, cresol red, pararosaniline, magenta red, xylenol blue, bromocresol purple, bromophenol blue, bromothymol blue, metacresol purple, thymol blue, bromophenol blue, tetrabromophenol blue, brom-chlorphenol blue, bromocresol green, chlorphenol red, o-cresolphthalein, thymolphthalein, metanil yellow, diphenylamine, N,N-dimethylaniline, indigo blue, alizarin, alizarin yellow GG, alizarin yellow R, congo red, methyl red, methyl orange, orange I, orange II, nile blue A, ethyl bis(2,4-dinitrophenyl) acetate, gamma-naphtholbenzein, methyl violet 6B, 2,5-dinitrophenol, p-nitrophenol, and/or the various functionalized derivatives of the above species. Even when an indicator cannot be bound unchanged with retention of indicator activity, one or more of its derivativates can often be bound with satisfactory results.

Bound redox indicators can be made from organic species which include methylene blue, diphenylbenzidine, diphenylamine, ethoxazene, and N-phenylanthranilic acid and/or suitable derivatives of any of these.

Bound adsorption indicators can be made from organic species which include fluorescein, diiodofluorescein, dichlorofluorescein, phenosafranin, rose bengal, eosin I bluish, eosin yellowish, magneson, tartrazine, eriochrome black T and others.

The following examples illustrate typical methods of preparation of the new indicators:


Indicators in the form of a stirring rod, and in the form of a powder, were prepared, starting with a heavily etched silica glass stirring rod for the former, and 1 gram of fragments of 96% silica porous glass for the latter. Both carriers were cleaned by soaking in 0.2M nitric acid at 95.degree. C for 1 hour, rinsing several times with distilled water and then heating overnight at 650.degree. in air.

The two samples of glass were cooled, placed in flasks and to each was added 50 millilitres of a 10% solution of gamma-aminopropytriethoxysilane. Both mixtures were refluxed overnight, cooled and washed with acetone.

The two glass products, now in the form of aminoalkylsilane derivatives, were refluxed for one hour in solutions containing 10 ml of chloroform, 100 mg of p-nitrobenzoyl chloride and 50 mg of triethylamine, washed with chloroform and air-dryed. The nitro groups were reduced by refluxing in 1% aqueous sodium dithionite, giving the arylamine derivative. The amino groups were diazotized by adding 10 ml of glacial acetic acid followed by an excess (0.3 g) of sodium nitrite. The mixtures were placed under vacuum until all air and gas bubbles were removed from the glass, after which 1 g of phenolphthalein was added to each followed by placing under vacuum for a further 30 minutes. The resulting products, which in each case was phenolphthalein coupled to the silane by an azo linkage with the silane bound to the glass, were washed with water, acetone and benzene until any phenolphthalein non-covalently adsorbed on the glass was not detectably eluted. The phenolphthalein glasses when exposed to liquids of different pH concentration underwent a colour change which I observed to occur in the pH range 8.5-9.0; at a pH of 8.5 the glasses were pale yellow and at a pH of 9.0 were deep red-brown. These indicators retained their colour and activity indefinitely, despite exposure to strong organic and aqueous acids and bases, to various solutions and reagents, and to air.


A 1 g sample of porous zirconia-clad silica glass amino alkylsilane derivative (e.g. Corning Glass Works product MAO-3930) was refluxed for 1 hour in a chloroform solution containing 100 mg p-nitrobenzoyl chloride and 50 mg triethylamine, as in example I. The nitro groups on this product were again reduced with dithionite. This was followed by diazotization by 0.3 g sodium nitrite in 10 ml glacial acetic acid, under a vacuum at 0.degree.. Excess (0.5 g) N,N-dimethylaniline was added. The product was a deep-burgundy colour in solutions of pH below 4.5, turning to a pale orange-red colour at pH 4.5 to 5.5 The bound indicator retained colour and activity, despite exposure to various conditions. Presumably, the bound indicator is of the following structure: ##EQU1##


A 15 g sample of 2,4,6-trimethoxybenzoic acid and 49 g trichlorosilane were dissolved in 200 ml acetonitrile and refluxed for 1 hour. Two equivalents of tripropylamine were added at this point and the resulting mixture was refluxed at 80.degree.-90.degree. for 8 hours. Treatment with dry ether caused the precipitation of tripropylamine hydrochloride (95%). Distillation of the filtrate gave 11 g of 2,4,6-trimethoxybenxyltrichlorosilane boiling at 80.degree.-84.degree. (6mm). This product was dissolved in 100 ml ethanol. Five equivalents of tripropylamine were added and the mixture refluxed for 1.5 hours at 70.degree.-75.degree.. The mixture was distilled yielding 2.1 g of 2,4,6-trimethoxybenzyltriethoxysilane. 1 g of porous glass and 20 ml of toluene were added to this and the mixture was refluxed overnight giving a trimethoxyarylsilane glass derivative.

Fifteen g of 3-nitro-N,N-dimethylaniline was mixed with 17 g of sodium thiosulphate in 200 ml water and refluxed for 1 hour to give 3-amino-N,N-dimethylaniline. This mixture was cooled to 10.degree. and sodium nitrite (20 g) was added slowly. 3-diazo-N,N-dimethylanilinium chloride was collected as a filtrate and added to the trimethoxyarylsilane glass derivative and 10 ml glacial acetic acid, in an ice bath. The mixture was evacuated for 30 minutes to remove air and gas bubbles from the glass. The reaction product, which was again N,N-dimethylaniline bound to glass by azo linkage to the silane, was washed extensively until molecules non-covalently adsorbed on the glass were not detectably eluted. The product was a bound indicator of the structure: ##SPC1##

In acidic solutions of a pH below 4.0 it was burgundy in colour and underwent a colour change to pale orange-red between pH 4.0-5.5. The bound indicator retained colour and activity despite exposure to various organic and aqueous solvents and to strong acids and bases. Exposure to a sodium hypochlorite solution caused irreversible colour change to pale yellow and hence its destruction as a useful bound indicator.


The procedure of Example II was used to produce, from the reduced form of methylene blue, methylene blue bound by azo linkage to zirconia-clad glass. The glass is bright blue under most solution conditions, but turns reversibly pale yellow when exposed to strong reducing conditions such as zinc dust in dilute sulfuric acid.


2,4,6-trimethoxybenzoic acid was dissolved in sulfonyl chloride and refluxed over a steam bath for 1 hour to give 2,4,6-trimethoxybenzoyl chloride. To 250 mg of this was added 10 ml of pyridine and 1 g of aminoalkylsilane porous glass (Corning GAO-3940). This mixture was stirred at room temperature, and then refluxed 30 minutes to give a glass complex with highly activated phenyl rings, presumably of the following structure: ##EQU2##

This glass was decanted, washed in acetone and air-dryed. The nitro group of Eriochrome Black T was reduced to an amino group, the product was recrystallized from ethanol, and: 1 g of this indicator derivative was added to 10 ml of glacial acetic acid over an ice-bath and diazotized with 0.5 g sodium nitrite. Then 1 g of the glass complex was added to this mixture. The resulting orange bound indicator was washed thoroughly with water, acetone and benzene until colour was no longer eluted. This orange bound indicator turned reversibly violet in the range pH 10.0 to 12.00.


Nickel screen, of 150 mesh and 0.1 mm thickness, was heated overnight in a furnace at 700.degree. in an oxygen atmosphere to oxidize the surface, thus forming a NiO coating on the screen. The screen was then cut into strips 1 inch by 4 inches, which were rolled into cylinders of approximately 0.5 inch diameter and the ends soldered to prevent ravelling.

One of these NiO coated cylinders was refluxed overnight in a 10% solution of gamma-aminopropyltriethoxysilane in toluene. This aminoalkylsilane derivative was washed in toluene and air-dryed. The screen was refluxed in 10% thiophosgene in chloroform. The isothiocyanoalkylsilane derivative was washed with chloroform and coupled to ethoxazene. The bound indicator thus created underwent a colour change from red below pH 5 to yellow above pH 5.


4-carboxy-alpha-hydroxy-alpha, alpha-bis (p-hydroxyphenol)- 1-toluenesulfonic acid was prepared by condensing phenol with 3-carboxy-1-sulfobenzoic anhydride in the presence of zinc chloride. Equimolar quantities of this phenol red derivative and tri-n-propylamine were combined with 3 molar equivalents of trichlorosilane in a vigorously exothermic reaction. After refluxing for 1 hour at 55.degree.-75.degree. and treating the mixture with tri-n-propylamine hydrochloride in pentane, and ethanolysis, the product, an indicator silane, was isolated by low pressure distillation.

This silane coupling agent, 3-carboxyphenol red triethoxysilane was then bound to 1 g of porous 96% silica glass as in Example I, to give methylenephenol red glass which was red at pH 7.0 and below and changed to orange-yellow at pH 8.5.


A bound indicator on a glass carrier was prepared by the procedure of Example I from fluorescein instead of phenolphthalein.


A bound indicator on a glass carrier was prepared by the procedure of Example I from xylenol blue instead of phenolphthalein.


A bound indicator on a glass carrier was prepared by the procedure of Example I from cresol red instead of phenolphthalein.

While a number of examples of the bound indicators of this invention and methods of preparing them have been given, such disclosure is intended for illustration only and to impose no limitation on the scope of the invention beyond those included in the appended claims.

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