[STM]

Hello everyone,

Please I need suggestions on how to make my experiment better.

I intend to generate singlet oxygen in the gas phase. I am working with a PM 2.5 filter treated with rose bengal gel and allowed to dry.
The basic idea is that in the presence of oxygen under irradiation, the rose bengal in the filter will sensitize the oxygen passing through the filter to singlet oxygen which will then be collected in a singlet oxygen trapping solution.

The trapping solution is contained in a 4 ml cuvette while the sample filter is placed on top the cuvette. The filter is attached to a glass column through which oxygen flows under air-tight conditions. The irradiation light is applied from outside the column directed to the filter.

So far, my trial has been unsuccessful as there is no difference in the response of the collecting solution using either treated or untreated filter (control).


Abs of rose bengal = 550 nm
Irradiation light : A white LED light (The conventional LED touch in convenient stores)
O2 flow rate : 0.1 to 2 L/min has been tried.
Distance between filter and collecting solution is < 5 mm.

Please kindly advice me on what I might be missing in this experiment that is responsible for its failure so far.

What can I do to improve the experiment and get result? The theory seems quite logical but I don't understand while its not working so far.

Thank you

[Corribus]

What is your "trapping" solution? Singlet oxygen has a very short lifetime in most solvents. (Your title indicates gas phase.)

[STM]

The trapping solution is singlet oxygen sensor green (SOSG) in 2% methanol

[Corribus]

In water, singlet oxygen has a lifetime on the order of a microsecond. You'll need a substantial concentration to see it.

You may check out: http://pubs.acs.org/doi/pdf/10.1021/ja00537a030

[Enthalpy]

Did you compute the amount of light compared with the oxygen amount?

Let's imagine that a strong white Led emits 20mW light of which 5mW are arount 550nm=2.25eV, that's 9×10^-8mol/s. I can't imagine any significant effect on 0.1l/min=7×10^-5mol/s oxygen. The energy towards singlet oxygen must originate somewhere, from the light I suppose.

Also, a very fine solid like a filter must be a catalyst for singlet oxygen decomposition - but I'm outside of my knowledge on that.

[STM]

@ Enthalpy: Thank you for your contribution. The energy for singlet oxygen generation from molecular oxygen is obtained from a photosensitizer, rose bengal in this study.

Rose bengal absorbs light (excitation wavelength, 550 nm) and becomes sensitized. It then transfers this energy to oxygen which becomes excited to singlet oxygen.

The idea is that in the rose bengal-treated filter under irradiation and in the presence of oxygen flow, the mechanism of Light energy-Rose bengal photosensitization-energy transfer to molecular oxygen-singlet oxygen generation should be feasible.

[Corribus]

There are a lot of potential issues. Probably the two that you should check out first are:

1. Concentration of sensitizer is not high enough.
2. Light intensity is not high enough.

Have you tried measuring the light intensity at the excitation wavelength?

@Enthalpy

Not a bad thought, re: filter oxidation. Singlet oxygen will react with just about anything. If the filter is made of organic material, as is likely... OP might try checking FTIR/ATR spectrum of filter before and after the experiment. If he/she observes a growth in the carbonyl stretching region, that might be at least a partial explanation.

It's still hard for me to visualize what the OP is doing. But given that the lifetime of singlet oxygen in aqueous solution is very small fractions of a second, you have to consider how far a singlet oxygen can diffuse over that timescale. It's possible it is being generated in substantial amounts, but by the time/place of a detection, there is none left to detect because it has decayed.

(You can observe singlet oxygen activity pretty easily in a time resolved absorption experiment, or time-gated fluorescence - but here you're not relying on the generated singlet oxygen to diffuse somewhere else - excitation and observation occur at the same basic spot - and the flash lamp gives you adequate time resolution to observe absorption or luminescence intensity changes associated with singlet oxygen generation.)

[STM]

@Corribus: Thank you for your helpful contributions.

Regarding what I am trying to do, A PM 2.5 filter paper was treated with Rose bengal solution. My aim is to see if singlet oxygen is generated from the filter paper under irradiation in the presence of oxygen.

My present experiment was designed with the assumption that the photo-generated singlet oxygen may diffuse through the pores of the filter along with the flowing molecular oxygen and collected in the trapping solution containing singlet oxygen probe compound.

Nevertheless, I am open to other ideas on experimental set-ups that can enable me monitor potential singlet oxygen generation from the surface of the rose bengal-treated filter paper.

Thank you

[Enthalpy]

My question expressed a doubt. Only rose bengal excited by light will produce singlet oxygen, unless I missed something, and the light power doesn't match at all the oxygen throughput. 1000* that light power would be a better start.

One other worry is that all the paper filter, unexcited rose bengal, solvent as Corribus pointed out, decompose singlet oxygen well before you get a chance to observe it. In fact, even gaseous singlet oxygen decomposes when two molecules meet, so that the concentration stays low.

But if you find a way to store singlet oxygen, pure or well concentrated in normal liquid oxygen, I want it to propel rockets.