In experiments, when the color of magnetic beads gradually changes from brown to reddish-brown, many people may wonder: can these magnetic beads still be used? In fact, the discoloration of magnetic beads is a common phenomenon with certain scientific principles behind it.
I.Reasons for Magnetic Bead Discoloration
Magnetic Silica beads commonly used in nucleic acid extraction have a typical core-shell structure. The outer layer of silicon dioxide (SiO₂) acts like a protective film, not only providing protection but also endowing the magnetic beads with surface functional groups. The inner magnetic core is composed of iron oxides, mainly a mixture of Fe₃O₄ and γ-Fe₂O₃, which is the key to the superparamagnetism of magnetic beads.
During storage, the magnetic core comes into contact with air and moisture, leading to an oxidation reaction: Fe₃O₄ (black) reacts with oxygen to form γ-Fe₂O₃ (reddish-brown). Simply put, Fe²⁺ in the magnetic core is oxidized to Fe³⁺, which is the direct cause of the color change of magnetic beads.
The color of magnetic beads is also affected by the magnetic core material, surface coating material, and particle size, among which the magnetic core material has the most significant impact. For example, black Fe₃O₄ turns into brownish-yellow Fe₂O₃ after oxidation. Most magnetic beads used for nucleic acid extraction have their surfaces modified with SiO₂, but this SiO₂ layer is not completely dense and has certain gaps. Small oxygen molecules can enter through these gaps, gradually oxidizing Fe₃O₄ to brownish-yellow Fe₂O₃.
II. Impact of Discoloration on Magnetic Bead Performance
(I)Impact on Structure and Magnetic Response
Many people worry that the saturation magnetization of magnetic beads will decrease significantly after oxidation. In fact, although there are differences, the decrease is not obvious. The saturation magnetization of Fe₃O₄ is indeed higher than that of the oxidation product γ-Fe₂O₃. After oxidation, the magnetic responsiveness of individual particles will decrease, but for conventional nucleic acid extraction operations, the superparamagnetism of γ-Fe₂O₃ is sufficient to adsorb magnetic beads to the tube wall. Experimental data show that the magnetic response time does not change significantly with storage time, indicating that oxidation has little effect on the magnetic response time.
Some may ask whether the silica shell will crack, leading to leakage of iron ion . The answer is yes. Although the silica shell has protective and functional effects, it is not completely without gaps, and the coating thickness of different magnetic beads also varies. During long-term storage, factors such as changes in buffer pH, temperature fluctuations, and mechanical stress may cause nano-scale cracks and pores on its surface. This can have two effects: first, providing channels for dissolved oxygen to accelerate the oxidation of the internal magnetic core; second, causing Fe²⁺/Fe³⁺ ions to leak into the surrounding buffer.
(II) Impact on Nucleic Acid Extraction Performance
In the early stage of slight oxidation, the impact on magnetic bead performance is limited, but long-term oxidation will reduce the nucleic acid extraction efficiency. On the one hand, the leaked Fe²⁺ and Fe³⁺ are high-valent cations, which will compete with target cations (such as Na⁺) for binding sites on the surface groups of magnetic beads and the DNA phosphate backbone, thereby reducing the nucleic acid adsorption efficiency. However, cleaning the magnetic beads before use can alleviate this problem to a certain extent. On the other hand, over time, cracks in the silica shell may worsen or even peel off, reducing the number of functional groups that can bind to nucleic acids. However, during the shelf life of magnetic beads, the change in surface carboxyl density is within a controllable range.
But it is reassuring that after the color of magnetic beads changes, their “working ability” is still reliable. It can be concluded from relevant data that the saturation magnetization remains at 60 emu/g before and after discoloration, and they still have superparamagnetism, enabling rapid magnetic recovery. As magnetic beads for nucleic acid extraction, their work efficiency is high, and the nucleic acid recovery rate can reach more than 90%. Even if some magnetic beads discolor after 14 days in a “high-temperature aerobic environment”, their work efficiency will not be affected.
III. Recommendations for Magnetic Bead Storage
The color change of magnetic beads from brown to reddish-brown is an obvious sign of magnetic core oxidation, and its impact on performance appears gradually. The impact is not significant in the early stage of slight discoloration, but as the aging degree increases (darker color), the risk of performance degradation will increase significantly.
However, there is no need to worry excessively. After magnetic beads oxidize and discolor, a dense oxide layer will form, and no further changes will occur. Therefore, this oxidation is safe and will not affect their working ability.
To keep magnetic beads in good condition, they should be stored strictly in accordance with the instruction, usually refrigerated and sealed, and attention should be paid to the expiration date. If obvious discoloration of magnetic beads is observed, it is recommended to verify the performance with a new batch of magnetic beads before processing precious samples.