1.
If you say that there is not an ~ or not one ~ of something, you are emphasizing that there is not even a very small amount of it.
He’s never shown an ~ of interest in any kind of work.
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QUANT: with brd-neg, QUANT of n-uncount emphasis
2.
You can use an ~ or one ~ to emphasize a negative statement. Not an ~ or not one ~ means not even to a small extent or degree.
Our credit standards haven’t changed one ~.
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PHRASE: with brd-neg, PHR after v emphasis