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Fluorescein & its Sodium Salt | Chemistry, Uses and Side effects

Home » Blog » Fluorescein Sodium » Fluorescein & its Sodium Salt | Chemistry, Uses and Side effects

What is Fluorescein

Fluorescein, commonly known as uranine, resorcinolphthalein, or D&C Yellow, is a dye derived primarily from two petroleum products, resorcinol and phthalic anhydride. It was first synthesized by von Baeyer in 1871. Fluorescein is a hydrocarbon that comes in the form of a dark orange/red powder that is only slightly soluble in water and alcohol.

Chemical Structure of Fluorescein

It is frequently utilized as a luminous tracer in a wide range of applications. It is available in a sterile aqueous solution in 2 to 3 mL concentrations of 25% sodium fluorescein or 5 mL concentrations of 10% FS. Approximately 80% of fluorescein dye is attached to plasma proteins, principally albumin, after injection into the circulation. The dye is metabolized by both the liver and the kidneys and is excreted in the urine within 24 to 36 hours.

Fluorescein Sodium is the most common commercially used salt of Fluorescein. The rest of the article discusses overall aspects of Chemistry, Mechanism of action, uses and side effects of Fluorescein Sodium.

Note for the reader >>> In a few instances this article refers to Fluorescein Sodium as FS.

Fluorescein Sodium

Some of the common synonyms of fluorescein sodium are Fluorescein (Sodium Salt), Uranine, Resorcinolphthalein, Nafluo, Acid Yellow 73,  D&C Yellow #8, and Yellow No. 8. It can also be identified by its CAS no. which is 518-47-8.

There exist other forms of fluorescein, except sodium salt but they are not known for their fluorescence. Dyes such as rose bengal and eosine are also derivatives of resorcinol and phthalic anhydride but they instead of sodium they are in combination with other substances like bromine or iodine.

Chemistry of Fluorescein Sodium

At 195 degrees Celsius, seven-part resorcinol and five-part phthalic anhydride are combined to make Fluorescein Sodium.

It is boiled, purified, precipitated, refiltered, solubilized, reprecipitated, and the process is done across several steps and with many solutions, most notably sodium hydroxide—so that it becomes a purified substance at the end of the procedure that can be used to make an injectable solution, a topical solution, or dye-impregnated paper strips. To create the injection form, water, sodium hydroxide, and/or hydrochloric acid are added to the powder.

Reference for above – Fluorescein Sodium What is it?

This molecule’s fluorescence is highly strong; peak excitation occurs at 494 nm and peak emission occurs at 521 nm. At 460 nm, fluorescein exhibits an isosbestic point (equivalent absorption for all pH levels).

Fluorescein sodium Chemical Structure

Its chemical formula is C20H14Na2O7.

Its IPUAC Name is disodium;3-oxospiro[2-benzofuran-1,9′-xanthene]-3′,6′-diolate;dihydrate.


Chemical Structure of Fluorescein Sodium


Property Description
Chemical name 3′,6′-dihydroxyspiro[isobenzofuran-1(3H),9′-[9H]xanthen]-3-one
Product name Fluorescein Sodium
Chemical Formula C20H10Na2O5
CAS No. 518-47-8
Molecular Weight 376.05
Appearance Orange Red, Fine powder, Hygroscopic
Toxicity Low toxicity
Density 1.602 g/mL
Melting point 314 to 316 °C (597 to 601 °F; 587 to 589 K)
Solubility Slightly soluble in water, highly soluble in ethanol
pH value 8.0 to 9.8
Odour odourless
Colour index no 45350
Maximum absorption wavelenght 493.5 m
Storage temperature +5°C to +30°C.
Pubchem CID 10608
Pubchem SID 462730481

Note – For injectable grade Fluorescein Sodium, both acids and alkali forms may cause tissue damage. Also when the solution is made neutral or acidic it loses its fluorescein property.

Following are some of its properties with regard to its identification:-

  • A solution of FS is strongly fluorescent, which disappears if the solution is made acidic and reappears if made alkaline.
  • When you place one drop of a solution on a piece of filter paper it produces a yellow stain, and when exposed to the vapour of bromine and then to vapour of ammonia for a minute it turns deep pink in colour.
  • After incineration of FS solution, the residue that remains will respond to all tests for Sodium.

Mechanism of action

Fluorescein sodium dissolves easily in aqueous alkaline solutions as an ocular revealing agent, reacts to cobalt blue light at 465 to 490 nm, and fluoresces as brilliant green at 520 to 530 nm. The chemical comprises a conjugated system that glows when electrons are stimulated for an extended period of time. As a result of being stimulated by blue light, the hydrocarbon produces light that appears yellowish-green. This is why Fluorescein Sodium is used in angiography.

During an angiogram, the fluorescein dye molecules absorb the blue light, which raises the electrons to a higher energy level. When this light ceases its emission, the electrons release some of the excess energy that they have absorbed and return to their normal energy levels. From the light that is released, ophthalmic photographers can then capture minute details of the eye, they otherwise could not have captured.

Reference for the above:  Fluorescein Angiography

Uses and benefits of Fluorescein Sodium

Uses of Fluorescein Sodium

Medical uses of Fluorescein Sodium

Fluorescein in topical uses is used in the diagnosis of herpetic corneal infections, corneal abrasions, and corneal ulcers. The salt with the sodium base known as Fluorescein sodium is mainly used as a tool of diagnosis in the fields of optometry and ophthalmology. It is used also to know the tear layer underneath the rigid gas permeable contact lenses during the fitting.

Fluorescein angiography, which uses oral fluorescein intravenous to categorise and diagnose vascular disorders like inflammatory intraocular conditions, macular degeneration, diabetic retinopathy, and intraocular tumours, is used in the research or categorise and diagnose vascular disorders.

During the open-heart surgery, diluted fluorescein dye has been used to find many muscular ventricular septal defects, it helps confirm the presence of any residual defects present.


In ophthalmology and optometry, fluorescein sodium is widely used as a diagnostic tool. It includes all from applanation tonometry, gonioscopy, and fitting of the contact lens to iris vasculature and retina angioscopy or angiography. After putting anaesthetic and fluorescein strips to the eyes, a tonometer is put on top of the cornea to understand the intraocular pressure of one eye at one time in the given first scenario. It is also used in applanation tonometry to make semicircles which help with the dial reading measurement. Fluorescein angiography stains the blood vessels of the retina and iris to give out a detailed image of the posterior view of the eye, which allows for better detection of any breaks that happen in the vasculature or other associated retinal conditions.


Fluorescein’s extraordinary fluorescence helps for the detailed understanding of tumour-affected tissues, non-diseased tissues, and histological markers. Fluorescein can also be used as an agent of contrast in surgical procedures to help non-specifically identify anatomic arrangements for example. The fluorescence of the focused region’s tissue, which is healthy, separates it from normal tissue nearby, allowing surgeons to repair or remove important tissues while protecting vital structures. Fluorescein can also be used in surgery of the hand to see the nerves needed for fine motor skills or to separate blood vessels in a specific section.

Conjugate Species/Biomarker

In immunohistological staining, fluorescent microscopy helps for knowing cellular components or microorganisms like proteins. Enzyme-Linked Immunosorbent Assay known as ELISA, the common indirect method of immunohistochemistry, uses fluorescein as a fluorescent conjugated to secondary antibodies in general. In a water-soluble chitosan derivative has combined with a fluorescent tag, the result was glycol chitosan fluorescein-isothiocyanate known as GC-FITC, which proved as a sufficient bio-imaging agent when it was compared to the commercial alternatives.

Fluorescein angiography

The medical procedure which involves the injection of fluorescent dyes into the bloodstream of the body is known as fluorescein angiography. This dye takes attention to the blood vessels in the backside of the eye, which allows it to be photographed.

The test is mostly put to use to diagnose and cure the problems. It can be ordered by your medical practitioner to confirm a diagnosis, narrow down the best treatment, or keep a check on the health of vessels in the backside of the eye.

Fluorescein angiography can be recommended by your practitioner to know if blood vessels in the backside of the eye are getting proper blood flow. It can also be put to use to help diagnose eye diseases like macular degeneration and diabetic retinopathy by your doctor.

Vascular Neurosurgery

Successful neurosurgery on vascular lesions needs the techniques to evaluate the blood flow. Vascular lesions like arteriovenous malformations, cerebral aneurysms and Dural arteriovenous fistulas, and also the other clinical scenes that need vascular anastomosis, have distinct flow alterations for example aneurysmal turbulent flow. If the flow of blood has been affected or the vessel is torn during the procedure, the clinical outcomes could be bad.

Biochemical research

In the applications of fluorescence microscopy, the isothiocyanate derivative of the fluorescein is mostly put to use to track and label the cells in the biology of cells. Fluorescein sodium can be also attached to different biologically active molecules like other antibodies, this allows biologists to focus the fluorophore on specified structures within cells or proteins. It is a very common yeast display application.

In the hybridisation of situ, it can be achieved by conjugating enzymatically fluorescein to nucleoside triphosphates and making it incorporated in a probe. Fluorescein-labelled oligonucleotides which are synthetic are put to use in different techniques labelled as beacons of the molecule.

Scientific Uses

Here are some scientific uses of fluorescein sodium.

Earth sciences

The Fluorescein sodium is also a flow tracer conservative which is mostly put to use in tests of hydrological tracer to help understand both groundwater flow and surface. This dye can also be put as a methylated spirit dye in places like New Zealand, Australia, and it also can be put in rainwater in the testing of environmental simulations to help in finding and understanding the water leaks.

Plant science

Fluorescein has been used for a long time to keep track of the movement of water in systems of groundwater to understand the flow of water and spot areas of obstruction or contamination. Because of the fluorescent dye, areas of problem turn out to be more visible and easier to see. Because this dye can make the problems in plant vasculature easily visible, a concept similar can be used on plants. Fluorescein and fluorescent dyes are used in plant science to keep in check and learn plant vasculature, mainly the xylem, which is the plant’s main water transport path.

Side Effects


The most common side effect of fluorescein is nausea, which is caused by a discrepancy in the pH of the body and sodium fluorescein dye; however, a variety of additional variable factors are also considered as the causes.  The nausea is generally temporary and goes away quickly.


Hives can range from a scale of mild to severe. Though an anaphylactic shock, followed by cardiac arrest and abrupt death, has been observed in extremely rare cases. Still, there remains a possibility because it can happen quickly in a moment, a fluorescein-using health care professional should be positioned to provide emergency treatment.

It can cause other side effects in a few cases still; nausea and headache are the most common. However, there might be some cases where the side effects become severe and require immediate medical attention.

Extravasation of the liquid at the injection site results in acute pain at the injection site as well as a dull aching sensation in the injected arm. There have been reports of generalized hives and itching, as well as bronchospasm and anaphylaxis. Following injection, an unusually strong sense of taste might emerge.

Read more >> Fluorescein (Intravenous Route)

Food or drug-induced urticaria, asthma, eczema, and hypersensitivity; those intolerance responses might not be identified by a fluorescein test, which is of no use and may even be lethal. A visit to an allergy expert may help in understanding the symptoms.

  1. Vision might become hazy causing things to appear dark or coloured. This negative effect subsides within a few minutes.
  2. Your skin may appear yellowish in colour. This occurs because the dye goes down to all of the veins throughout the body. Though in a few hours, the skin returns to its normal colours.
  3. Urine may appear orange or dark yellow for about one day following the treatment. This is due to the fact that the kidneys take time to filter the dye from the blood.
  4. If dye spills during the injection, it may cause a skin burn. This negative effect subsides within a few minutes.
  5. Other side effects include gastrointestinal reactions, cardiopulmonary reactions, neurological reactions, hypersensitivity reactions and thrombophlebitis.

Side-effects of Fluorescein Sodium Injection

Fluorescein Sodium injection, in some cases, can cause significant intolerance responses that usually can’t be predicted but are more frequent in patients who have had a poorly tolerated injection (except for symptoms of nausea and vomiting that are common) or who have a history of an allergic response to the drug.

Rare Side-effects of Fluorescein Sodium Injection which requires immediate medical attention

  • Difficulty in breathing for a prolonged period
  • Discolouration of the skin persisting for more than 24 hours
  • Difficulty in swallowing food
  • Severe dizziness
  • Fastened heartbeat
  • Swollen eyelids
  • Severe pain in the arm and unusual fatigue

Common Side-effects of Fluorescein Sodium Injection which do not require immediate medical attention

Certain fluorescein side effects may happen after a few minutes or even hours after its administration. However, they usually do not necessitate medical treatment. These adverse effects may subside as your body responds to the medication. In addition, the health care provider may be able to advise on how to avoid or mitigate some of these adverse effects.

  • Puking
  • Discolouration of skin
  • Abdominal pain

Side-effects of Fluorescein Sodium Ophthalmic solution

Transient painful burning sensation and tearing combined with conjunctival chemosis and yellowish discolouration are possible ocular adverse effects of intravenous fluorescein dye. Through an unknown mechanism, eyes with active inflammatory disorders may be prone to this unusual outcome.

Rare side-effects of Fluorescein Sodium Ophthalmic solution which require immediate medical attention  

  • Hindered blood supply to the brain
  • Bronchospasm
  • Itching
  • Seizures;
  • Anaphylaxis
  • Thrombophlebitis

Common side-effects of Fluorescein Sodium Ophthalmic solution which do not require immediate medical attention

  • Low BP
  • Headache
  • Vomiting
  • Upset stomach

Important precautions one should be aware of before using Fluorescein Sodium

Due to the risk of hypersensitivity responses, the following precautions should be taken throughout the process:

Read more – Things to keep in mind before using this medication

  1. The ophthalmologist doing the surgery must monitor the patient during the treatment and for half an hour after it is completed to check for any side effects that might occur.
  2. Medical access should be maintained for at least 15 minutes so that any mishap may well be handled as soon as it occurs.
  3. The prerequisites for immediate treatment must be easily accessible, depending on the insertion of a secondary venous line to enable vascular filling (polyionic solution or colloidal plasma replacement) and the IV administration of a recommended dosage of adrenaline.
  4. Furthermore, if a patient is identified as being at risk yet surgery is required, it must be conducted in the presence of a specialist, particularly if the patient is receiving beta-blocker medicine, including eye drops such as adrenaline.
  5. Due to the alkaline pH of the liquid, fluorescein solution can cause trouble when administered outside of the vein; It is essential to confirm that the needle is properly inserted into the vein before injecting the fluorescein injection. If the medication spills into the surrounding tissues, the infusion should be promptly halted.


Where to buy fluorescein sodium?

If you are buying fluorescein sodium for ophthalmic or pharmaceutical use we recommend you to buy it from a GMP compliant manufacturer who supplies FS in pharma grades like USP, PH-EUR. Please check with the supplier once that the material should not be from dye grade, as there are many instances in the industry of suppliers who sell dye grade FS for pharmaceutical use.

Macsen Laboratories, a unit of Macsen Labs, is one of the leading cGMP and US FDA compliant manufacturers of Fluorescein Sodium API.

Is sodium fluorescein safe?

Fluorescein has been used in medicine for many years to detect ophthalmic lesions including ulcers and degeneration of the cornea in humans for diagnostic reasons and is generally safe, however, large dosages of intravenous bolus injection may produce significant adverse effects. Nausea, puking, headaches, drowsiness, and low blood pressure are some of the side effects of intravenous injection.

Does fluorescein affect vision?

The vision may be distorted for a few minutes after the administration of the injection. It might appear dark for some time.

Can you drink fluorescein?

Fluorescein taken orally lowers the likelihood of getting allergic responses significantly. There have been no significant responses documented. However, minor reactions may also include itching or slight skin rashes.

Could you be allergic to fluorescein?

This medication has the potential to trigger a severe allergic reaction known as Anaphylaxis, which needs prompt medical care. The most severe symptoms of this response include rapid or erratic breathing, struggling to breathe, or fainting. Skin discolouration, a speedy yet irregular heartbeat or pulse, hive-like swellings, and puffiness of the eyelids or around the eyes are all possible symptoms. If any of these adverse effects occur, seek immediate medical attention.

When is fluorescein used?

Fluorescein is utilized to assist in the diagnosis of a variety of eye conditions. It is used to diagnose eye injuries and corneal abrasions when applied to the surface of the eye. It helps assess the blood vessels at the back of the eye during a fluorescein angiography, whether administered orally or by injection as an IV.

Updates from Macsen Labs on API- Macsen Drugs granted a patent from the Indian Patent office for Novel Fluorescein Sodium Synthesis Process

Other articles on Fluorescein Sodium


The information provided here is based on general knowledge, articles, research publications etc and we do not claim the authenticity of any of the information provided above. We do not claim or suggest/advise any medical, therapeutic, health or nutritional benefits of FS. We do not supply or promote our FS product for the applications which are covered by valid patents and which are not approved by the FDA.

Macsen Labs is a manufacturer and supplier of several grades of Fluorescein Sodium such as:-

Fluorescein Sodium USP

Fluorescein Sodium BP/ Ph Eur

Acid Yellow 73 (Fluorescein Sodium)

Solvent Yellow 94 (Fluorescein Free Acid)

Fluorescein Sodium API Manufacturer & Supplier