Optical Ion Sensor

Cheri W. Clavier, Yihui Yang, Alicia Vogt, D. Lynn Rodman, Joseph F.

Sinski, Hee-Jung Im, and Ziling Xue*

Department of Chemistry, University of Tennessee

Knoxville, TN 37996

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AbstractHeavy metal ions are used in a variety of industrial applications and many are considered pollutants.

Currently, industrial metal ion concentration measurements are mostly determined off-line, using techniques that are usually costly, time-consuming and inconvenient.

We are developing an optical ion sensor through the sol-gel process that will give concentrations of metal ions. Initial work has been carried out to explore the basic properties of sol-gel glasses with various amounts of amine ligand incorporated directly into the sol-gel matrix. The sol-gel preparation process was modified to produce optically transparent monoliths containing either the ethylenediamine derivative NH2(CH2)2NH(CH2)3Si(OMe3)3 (TMSen) or the monoamine ligand HN2(CH2)3Si(OMe)3 using Si(OMe)4 as a cross-linking agent. We have developed a novel gel-preparation process that gives strong, optically transparent monoliths without cracking. In addition, a sensor holder has been designed in a flow cell system that allows spectrophotometric measurements of the sol-gel disk in flowing Cu2+ solutions.

The kinetics of cupric ion uptake by the gels in solutions of various Cu2+ concentrations was studied by measuring the change in the absorbance of the metal:ligand complex over time using UV-VIS spectrophotometry. The gels are regenerated with acid or EDTA, neutralized, and then used in subsequent Cu2+ uptake/ removal cycles. Our studies thus far show the potential of the system in metal ion sensing applications.

We are currently focused on improving the gel regeneration procedure to provide reproducible second, third, and fourth cycle results.

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Motivation for Project Cu2+ is used in a variety of industrial applications

such as microelectronics, piping, and electroplating;

Cu2+ is recognized by the EPA as a toxic metal pollutant;

Concentrations of Cu2+ in industrial waste can vary widely, ranging from 0.5-500 ppm;

Currently, Cu2+ concentration measurements are determined off-line.

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Purpose - Long Term Goals

Chemically graft ligands onto sol-gel matrices and prepare transparent monoliths suitable for optical analysis;

Evaluate the spectroscopic response of such ligand-grafted sol-gels to target metal ion concentrations;

Study the stability and durability of the sol-gel sensors;

Test in flow systems;

Investigate the influence of other chemicals.

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Related Studies Sol-gel probes (single-use only) have been reported for the

determination of other heavy metal ions;1

Diamine (TMSen) anchored sol-gels have shown selective intake of Cu2+ ion with favorable kinetics and easy removal;2

Optically transparent, stable sol-gel monoliths have been reported using bis-TMSen anchored sol-gels for the analysis of proteins.3

(1) Oheme, I.; Wolfbeis, O.S. Mikrochim. Acta 1997, 126, 177-192 (2) Im, H.-J.; Yang, Y.; Allain, L. R.; Barnes, C. E.; Dai, S.; Xue, Z. Environ. Sci. Technol. 2000, 34, 2209-2214

(3) Rao, M. S.; Dave, B. C. J. Am. Chem. Soc. 1998, 120, 13270-13271

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Our Approaches Select ligands with good binding affinities for target metal ions,

such as ethylenediamine NH2(CH2)2NH-(CH2)3Si(OMe)3 or monoamine NH2(CH2)3Si(OMe)3 derivatives for Cu2+;

Incorporate the ligands into a sol-gel using Si(OR)4 (TMOS) as a cross-linking agent. NH

Cu2+

H2N

NH2HN

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The Sol-Gel Process Hydrolysis

Condensation

Functionalized ligand, L-Si(OR)3, can be anchored into the sol-gel matrix;

The ligand, L-, is covalently bound to the gel; Ligand leaching is not a problem.

Si OR OH Si Si O Si ROH++

Si OH OH Si Si O Si OH2++

Si OR OH2 Si OH ROH+ +

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Novel Gel Preparation Current Method

Form gel disks free of amine ligand;

Graft ligand in the 2nd step;

Use ethylene glycol to enhance gel strength;

Develop base-catalyzed process.

Advantages Fewer cracks; Stronger gel disks for

flow cell operation and Cu2+ uptake-removal cycles;

Optically transparent sensors.

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Photos of a Cu2+ loaded gel (left)and a blank gel (right)

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Characterization Studies

Brunauer-Emmett-Teller (BET) Gas Adsorption

Narrow pore size distribution;

Average pore diameter: Blank Gels: 45 Å Ligand-Grafted Gels: 67 Å

Fast kinetics.

Pore Size Distribution

Pore Diameter (Angstroms)

0 100 200 300

Pore

Vol

ume

(cc/

g)

0.0

0.1

0.2

0.3

BlankLigand-Grafted

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Flow-Cell Design

Accommodate and hold gel disks of various thickness steady in flowing solution;

Support two fiber optic cables that are isolated from the solution;

Interface with Ocean Optics S2000 fiber optic spectrometer.

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Flow cell with a Cu2+ loaded gel

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Spectrometer-Flow cell

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Preparation:Diamine-grafted gels

OHOHHO

Cu2+

NH2HN

NHHN

Si(OR)3NH2H2N

Si(OR)3]22+

NHH2N

Cu2+ NH

HN

N

N

Blank SiO2 Gel

Cu[

Solution

Blank Gel

Ligand-Grafted Gel

Si(OR)4 /

Cu2+ + 2

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Change in Absorbance over Time(Diamine-grafted gels, 1st cycle)

t (min)

0 2 4 6 8 10 12

A (

617

nm)

0.0

0.5

1.0

1.5

[Cu2+], mM

12.50

25.00

50.01

99.98

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Correlation between slopes dA/dt and [Cu2+](Diamine-grafted gels, 1st cycle)

[Cu2+], mM

0 20 40 60 80 100

dA/d

t

0.0

0.1

0.2

0.3 y = (2.82E-7)x3- (1.83E-5)x2 + (2.39E-3)x - 6.62E-3R2 = 1.00

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Regeneration:Diamine-grafted gels

Procedure: Cu2+ is easily removed from the gels with 1.0 M HCl; 0.010 M NaOH may be used to neutralize the gel.

Gel regeneration could be done on-line using the flow cell;

Abs vs. time plots change from first to second cycle.

+ +

++

NH2H3N

NH3H2NHN NH2

H2NCu2+

NH

H+ Cu

2+

Cu2+

/adjusting pH

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Reproducibility Study:Diamine-grafted gels

0.0777 ± 0.012 One cycle 50.01 mM [Cu2+]

Gel Slope (dA/dt)A 0.0890B 0.0763C 0.0843D 0.0612

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Results:Diamine-grafted gels

Showed a correlation between slope dA/dt and [Cu2+] for the 1st cycle;

Could be regenerated with acid; Neutralization with base was difficult; Results not reproducible from the 1st to 2nd

cycle; Potential use as a disposable Cu2+ probe.

20Preparation:Monoamine-grafted gels

OHOH

Cu2+ + Si(OR)3]42+4 NH2 Si(OR)3 NH2 Cu2+

NH2-

-H2N

-H2N

NH2-

NH2NH2Cu2+

NH2

H2N

H2N

NH2NH2 NH2

Blank SiO2 Gel

Cu[

Solution

Blank Gel

Imprinted Gel

EDTA

Ligand-Grafted Gel

Si(OR)4 / HO

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Change in Absorbance over Time(Monoamine-grafted gels, 1st cycle)

t (min)

0 2 4 6 8 10 12 14

A (

730

nm)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

[Cu2+], mM

5.00125.0050.01

75.03

99.98

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Correlation between Slopes dA/dt and [Cu2+](Monoamine-grafted gels, 1st cycle)

[Cu2+], mM

0 20 40 60 80 100

dA/d

t

0.00

0.02

0.04

0.06 y = (8.28E-8)x3 - (8.97E-6)x2 + (6.54E-4)x + 7.72E-3R2 = 0.999

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Reproducibility Study:Monoamine-grafted gels

Five cycles 75.03 mM [Cu2+]

Gel Slope (dA/dt) +/- A 0.041 0.005B 0.040 0.003C 0.039 0.003D 0.039 0.002

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Comparison - Diamine vs. Monoamine Gels

Diamine gel Two-step

preparation; Not reproducible

from the 1st to 2nd cycle;

Metal ion probe - one time use.

Monoamine gel Four-step imprinting

preparation; Reproducible over at

least 5 cycles; Metal ion sensor for

multi-cycle use.

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Results Prepared transparent sol-gel monoliths as substrates; Successfully grafted amine ligands into the sol-gel matrix; Designed a flow-cell coupled to a portable spectrophotometer; Evaluated the spectroscopic response of ligand-grafted sol-

gels to target metal ion concentrations; Demonstrated the application of diamine ligand-grafted gels

as a disposable Cu2+ probe; Illustrated the use of monoamine ligand-grafted gels as Cu2+

sensors for multi-cycle use.

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Future Plans Investigate the use of new, more sensitive ligands for

different metal ions; Explore thin film sensor approach for faster response; Study the influence of other chemicals; Explore “cocktail” approach for multi-component

analysis; Conduct field tests; Explore on-line regeneration of sensors.

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Acknowledgements

Measurement and Control Engineering Center

University of Tennessee, Knoxville